Beyond the people who's files have been crashing and commiting suicide, there has been plenty of time to complete a 10-20 second animation. Delays and sudden "sorry I can't finish" replies at the deadline always occur and are rather hard to avoid. The only thing you can do is periodic checkups after a week(or even a few days) to see if they made progress. If they are "too busy" to have done anything by then, then they should be dropped.
I mean if someone actually has a good solid start with over half the animation done, then I don't mind waiting a bit. But some people like to drop without showing any progress, It gives me the impression they "ASKED" for a spot and never even tried.
If there is an anime collab 2, I'd suggest being a hard-core ass and making sure that people are getting their work done at a reasonable pace. Because I've noticed that many of the newer collabs are being delayed for several month's, for no reason beyond just a "few" holding it back (due to lazyness, not flash crashes ;D )