Have you thought about going to have a chat with your doctor about it?
I know that if you've been on the pill for a long time (ie years) it can take a while to get it out of your system but I don't think you're old enough for that to be the problem.
I would suggest going and seeing what your doctor thinks, he may advise doing a sperm test on your fiance to see if they're active enough.
Good luck and let us know how you get on