ever hear the expression about programmers and herding cats?
programmers are difficult to manage, and even more difficult to motivate... the motivation has to come from within themselves... from an excitement about the project...
Make sure that your objective is clear - about what you want this end result to be - and be sure that they really want to do it...
I'm not trying to sound mean here - I just can't figure out how to say this nicely - -maybe the other programmers aren't interested in your project because it's just not interesting to them... ask for input from them... to make it more interesting or to give them a sense of ownership of it - where they will get motivated to make the code as good as it possibly can be.