I agree with Caligurl, it could be the pedals. When I first started mtbing my then fiance made me do it clipless. I started with the shimano ones my bike came with. I fell sooo many times. We changed the tension but it didn't work. I had a really hard time clipping in and out. I had to take my shoe off on the trail a couple times because I couldn't unclip. Finally, I bought a used pair of Time Atacs and my falling problem stopped. If you're having a really hard time clipping in and out, you may want to try a different brand. Once I switched, it fixed everything and I can't even imagine riding without them.