I experienced some difficulties with thinned epoxy drying as well. When I asked my friend/mentor about it, he indicated he'd never had the problem.

I finally solved the mystery with some research and testing and I found that the use of regular rubbing alcohol which is 30% water was causing me grief. The 91% concentration (Isopropyl) worked better but it's still got 9% water in it. The solution is to go to the local hardware store and buy you some denatured alcohol, then you can thin the epoxy as much as you want and it'll still dry...