Everyone here has given you wonderful advice thus far! Honestly there isn't too much more to add. The fact that you notice and are acknowledging that you might have a problem is also an amazing first step into setting things on a proper path.
I lead a plot driven FC of 20ish members, we've found that smaller groups are best if you want the story to be meaningful. We are undeniably a picky group (for better or for worse). I have two other leaders that have full access to the plotline and help shape and move the story forward for everyone. We try to adjust the path as things happen, but overall have a planned outline. It's always our goal to have as many people involved in the stories as possible since our size is pretty manageable.
Hype does wonders. You and your officers being excited is important. If you aren't excited, or don't show that you are, why would anyone else be? When we plan a major event we make a post about it on our forums, tack a message in our FC message thingy, sometimes we will drop small things into RP in the days leading to the event, and on the day of the event spam Discord so that everyone remembers. One of our leaders also makes nice little banners to pin to the messages, too. Time put into planning and advertising can really help.
Trimming the fluff would probably be a good thing. These people joined your LS for this certain story and type of RP, right? If they are no longer interested, they need to move along. No harm, no foul. The best thing that you can do to encourage RP and prevent from becoming a social or purely slice of life/tavern group is to keep like-minded people in the LS.
Do you keep a site for things? What about a Discord or Skype group? There isn't a day that goes by where we don't use Discord for chatting, RPing, planning, etc.
Does everyone get along OOC? A huge part of our recruitment criteria has to do with how well a potential member will get along with the rest of the crew. (again, this is extremely important because we are on the smaller side). People are more likely to jump in on events when they are invested in the cause, the characters involved, or simply to support the people running it. We also poke eachother on Discord all the time - 'Hey, does anyone want to do this or that?' 'I want to RP!' 'Shit's going down, who's with me?'
Sometimes being a leader is a thankless job. Keep lines of communication open with everyone. Have meetings (IC and OOC) with your group. Be open for change, and be ready to enforce rules if you need to. I personally recommend having 1 leader per 10-15 members. Always have someone around to answer questions, handle situations, etc. My leaders (I use this term loosely, the three of us are all on equal ground) and I talk about EVERYTHING. If there is a problem that one of us catches wind of, we bring it to leader chat and figure out how to best resolve the issue. Work together to figure out how to get over this RP dry-spell. The three of us collaborate and bounce ideas off of one another on our FC story and how to run each event. We offer to help with any events that our members want to run, as well.
There will be times where not everyone agrees. There will be times where members feel like you're ignoring them, and vice versa. Again. Communicate as much as possible. When (and you really should) you go to prune membership, give everyone advanced notice of your intentions. "In 30 days we will be cutting down on our numbers. People that don't RP will be removed. If you still want to be here, show us that you do!" Have some sort of 'active member' policy and keep it enforced. There can always be exceptions, ie 'RL always comes first, let us know if something comes up - we will put you on a special no-clean list'. Make your intentions known and clear, discuss them with your leaders and membership in an OOC meeting. Hold an IC one, too, if that's how you do things.
Really the best way to lead is to learn from the past. Try things out, and if they don't work change them just a little. Tweak things until you find what is just right for your group. Don't let yourself be too discouraged! Keep at it
I lead a plot driven FC of 20ish members, we've found that smaller groups are best if you want the story to be meaningful. We are undeniably a picky group (for better or for worse). I have two other leaders that have full access to the plotline and help shape and move the story forward for everyone. We try to adjust the path as things happen, but overall have a planned outline. It's always our goal to have as many people involved in the stories as possible since our size is pretty manageable.
Hype does wonders. You and your officers being excited is important. If you aren't excited, or don't show that you are, why would anyone else be? When we plan a major event we make a post about it on our forums, tack a message in our FC message thingy, sometimes we will drop small things into RP in the days leading to the event, and on the day of the event spam Discord so that everyone remembers. One of our leaders also makes nice little banners to pin to the messages, too. Time put into planning and advertising can really help.
Trimming the fluff would probably be a good thing. These people joined your LS for this certain story and type of RP, right? If they are no longer interested, they need to move along. No harm, no foul. The best thing that you can do to encourage RP and prevent from becoming a social or purely slice of life/tavern group is to keep like-minded people in the LS.
Do you keep a site for things? What about a Discord or Skype group? There isn't a day that goes by where we don't use Discord for chatting, RPing, planning, etc.
Does everyone get along OOC? A huge part of our recruitment criteria has to do with how well a potential member will get along with the rest of the crew. (again, this is extremely important because we are on the smaller side). People are more likely to jump in on events when they are invested in the cause, the characters involved, or simply to support the people running it. We also poke eachother on Discord all the time - 'Hey, does anyone want to do this or that?' 'I want to RP!' 'Shit's going down, who's with me?'
Sometimes being a leader is a thankless job. Keep lines of communication open with everyone. Have meetings (IC and OOC) with your group. Be open for change, and be ready to enforce rules if you need to. I personally recommend having 1 leader per 10-15 members. Always have someone around to answer questions, handle situations, etc. My leaders (I use this term loosely, the three of us are all on equal ground) and I talk about EVERYTHING. If there is a problem that one of us catches wind of, we bring it to leader chat and figure out how to best resolve the issue. Work together to figure out how to get over this RP dry-spell. The three of us collaborate and bounce ideas off of one another on our FC story and how to run each event. We offer to help with any events that our members want to run, as well.
There will be times where not everyone agrees. There will be times where members feel like you're ignoring them, and vice versa. Again. Communicate as much as possible. When (and you really should) you go to prune membership, give everyone advanced notice of your intentions. "In 30 days we will be cutting down on our numbers. People that don't RP will be removed. If you still want to be here, show us that you do!" Have some sort of 'active member' policy and keep it enforced. There can always be exceptions, ie 'RL always comes first, let us know if something comes up - we will put you on a special no-clean list'. Make your intentions known and clear, discuss them with your leaders and membership in an OOC meeting. Hold an IC one, too, if that's how you do things.
Really the best way to lead is to learn from the past. Try things out, and if they don't work change them just a little. Tweak things until you find what is just right for your group. Don't let yourself be too discouraged! Keep at it