If you wiki, I will follow??

I have been curious about developing ways to use wikis in my classroom. But today was the end of one of my units and they had to do a presentation to a group of "investors". I told them that they needed to have the powerpoint to me before class (emailed on Sunday, brought to me yesterday). In one class with 10 groups, I had 4. I gave them every opportunity to get this to me. If I can't get them to turn in this work, how can I expect them to take time outside of class to go online and post?