This is a topic near and dear to my heart, because I personally struggled with it for at least the first year I spent on LJRP, if not longer. I will say it upfront: it’s hard to write a post that gets lots of responses. Even with practice it can still be hard to do, mostly because so many different factors go into what makes a post successful. Some of these factors are things I can’t help you with, such as the popularity of your character or how active other players are when you make your post. But there is one thing I can advise you on, and that is the content of your post itself.
The first thing you need to know is that a great post always has a hook. It’s usually near the end of the post, and it is the thing about your post that catches everyone’s attention. A good way to imagine a hook is as the eye-catching detail that nobody can ignore. It could be something as simple as bumping into a person (which is hard to ignore because it’s a physical action against their character) or as complicated as riding down the street in a flaming rowboat that is being pulled by two polar bears (which is hard to ignore for obvious reasons). But regardless of how flashy that action is, it has to be a direct action that other players can easily respond to.
Before I continue, I would like to take a moment here to talk about the difference between passive roleplaying and active roleplaying. Passive roleplaying is when a player makes most of their posts as responses to what another player is doing, whereas active roleplaying is where a player has most of their responses being direct actions, usually done in response to a passive player’s responses. A perfect example of this is the all-too-familiar “reading in the library” post: the character reading the book is being passive, while the character who walks up and starts the conversation is being active.
Ideally, when you’re roleplaying a thread, no one character should be entirely active or entirely passive for the whole thread. Each character should be doing some of the actions, and they should also be responding to what the other character does. The reason for this is that active roleplaying takes a lot more effort than passive roleplaying, and it is really easy to get tired of a thread if you are constantly responsible for moving the action along.
Coming back to the original topic at hand, the reason most posts fail is that they were written to be passive roleplaying, which puts all of the effort of the thread onto whoever is tagging into the thread. The fact of the matter is, when you are making a post, you have to put the effort into that post. Passive starting posts are very easy to make, so they are a dime a dozen. Why should any character tag your specific “sitting in a library” post when there are five other “sitting in a library” posts right there alongside yours?
To sum it up, for your posts to get lots of responses, you need to have your characters commit some kind of action that other characters can easily respond to. Instead of sitting in the library and reading a book, have your character knock over a bookshelf while trying to get a book to read. Or have them be chased out of the library for being too loud. Or have them approach everyone else and say “hey, what are you reading?”
For practice, go to the main community for the game you are in and look at the last page or so of posts. Look for posts that have significantly more replies than others. Did that post have a good hook? What made that post interesting to respond to? Obviously other factors may affect this, but try to look for examples of active roleplaying versus passive roleplaying.