#1 reason for me is when I make a new character and zero plots happen. Of course, there's always the old "open thread" trick, but you can only do so many of those, and they're not necessarily going to be replied to. If I join a community, make a plotter page for my character that includes several plot ideas, reply to 4 different plotters, and don't receive any responses for more than a week as a result, I'm out. This includes if no one is talking plots/throwing ideas with each other in the chat.

I find that most of the time when this happens, the majority of the people who have joined are just dropping like flies--hence they're gone before they can see I replied to their plotter. That, or they're looking for very, very specific plots that they don't think will work with my character and/or only threading with people they're already friends with.

It's a very frustrating experience to put effort into a character and reaching out to other rpers with ideas, only to end up sitting there twiddling your thumbs. And there's only so much effort I'll put into chasing after plots or threads: somebody else needs to meet me halfway, or else roleplaying with them isn't going to work in the first place.