I've said it before and I'll say it again, but my biggest quip in regards to what can make or break an RP thread is dedication, commitment, and the willingness to do improvisation.
Dedication and commitment are vital when it comes to keeping threads going; If people/players/GMs don't try to act often enough or take very long with their turns, it holds things back. Get held back too much, and interest may be lost. Granted, this is pretty much a straightforward thought.
Willingness to improvise..... ANYTHING can be salvaged, ANY situation can be dealt with in some way. Any time anyone feels stuck, be it a player or a group, it helps to just wing it and get moving. A bit too often do I hear the words "I don't know what to do" or "I don't know how to do X" or "My character normally wouldn't do X". We're playing PRETEND to begin with using certain characters, the only thing limiting your options is yourself. Brainstorm and squeeze out any ideas you can come up with that may influence a character or situation to become unstuck; However absurd it may seem... I mean come on, we're mainly doing RPs based off My Little Pony; Surely we can be more lenient about unusual things being less unusual for the sake of progress once in a while.