Does anyone have experience dealing with copywriters?

I'm working with 2 at the moment who both started out great, but became unreliable after a couple of months.
Both complain to me that they have no money, but at the same time they keep slacking off, missing deadlines and turning down work I offer them.

What can I do?

They're both really good (when they apply themselves), but getting work out of them is proving difficult.