One of the issues is that many of the group games, particularly system games has an ol' boys network feel. I'm not saying anyone plays favorites, but I've seen the same names appear regularly, then disapper again for a variety of reasons. Sometimes, they give reasons, sometimes they just stop acting and this slows the game, if not bring it to a complete stand still as everyone waits for a response that doesn't come.
more care should be taken to look at a players posting history for consistancy as well as character concepts. In the end, you can have a group of great characters, but if your players aren't reliable and consistant posting, your game is going to fall to the wayside.
As far as those who disappear.. I don't think anything can be done except to try to select players who have either a history of consistant posting in the long run, or keep a list of those who don't and just not accept them until they change their patterns. In either case, 25% loss is to be expected and planned for. Overstocking a group in expectation of losing a third doesn't solve the problem, it just means that you still have more players in a game that may, or may not make it to the 4 month mark.