"I've tried lots of things to motivate them and didn't succeed." "I have to nudge them to get things done,constantly."

Sounds like the managerial and systemic failure is all yours, dude.

If you've not introduced automated systems to track what's being done and /help/ them (e.g. redmine, tracking relatively highly decomposed tasks, etc) keep on top of the work and the deadlines (and know why each deadline's important; i.e. who they'll be screwing over if they don't deliver) then you've only got yourself to blame.

Why would you think it'll be different with new people - because you'll just "be better" at hiring this time around? Unlikely.

J