MangaMan

Make sure the messages are actually reaching everyone. Before you send a private message on OCR, check the person's profile to see if they've logged in recently. If not, you might have better luck with a different method of communication, like email. I had to look up a guy's name and message him on facebook to remind him that he'd signed up for Essence of Lime, and he ended up following through.

As for deadlines themselves, I've always been pretty lenient about them. As long as people are keeping up-to-date and responding to messages, they'll probably get around to doing their work sooner or later (or they'll apologize and withdraw from the project). No mercy for the people who completely ignore the director, though. I've always managed to get in touch with unresponsive people by trying different methods, but if you can't get a reply despite jumping hurdles to contact them, drop 'em.