Emily:
I had a similar problem clipping into my Zero's also (and I weigh about 130). Have you tried loosening the 4 little screws on the cleats (as per the instruction sheet that came with the pedals)? They shouldn't be screwed down too tightly. Loosening them just a little bit solved the problem for me. As an added measure you can use LockTite (or something similar) on the screws.

Carol