Correct me if I'm wrong, but is this the core of the problem?
"I contract people to do a set amount of work at a fixed rate, and have no problem getting them to work for me. After that's done I offer them a new contract where they continue working for an unspecified time and not get paid at all, but for some reason they refuse to do it. What is wrong with them?"
If this is really what you're asking, and you actually have to ask it, then I think I see where the problem lies.