I have read elsewhere that some projects WU's are very dependant on completed WU's results and so can't issue new WU's until they get completed ones back. This makes it look like they are running low on work, but in actual fact you completing a task allows for another one to be issued, so you would be unlikely to run out work.

I think gpugrid is one these types of project, but i'm not sure on others. Does anyone else know?