I am having the same problem.....and never thought about pollination, silly me. I will bet that is what is going on.
I have a second garden in a church community garden, and planted zuchinni and it seemed a little slow also, but everyone else seems to be getting lots of beans there. We didnt plant any in that garden, so that doesnt help. Ah well, I actually BOUGHT some from the farmers market....gotta help those people too.