Not saying this is your problem for sure, but how's your butterfly stretch? Sitting with the bottoms of you feet touching, can you lower your knees to the floor? How about if you pull up on your feet just a little?
Based entirely on one experience, so take this with a grain of salt, a guy I was training with couldn't ohs or snatch because his body just folded in half at the bottom of his squat. One day when we were stretching, I noticed that his butterfly stretch, his knees were practically vertical. I taught him PNF stretching, he worked on his butterfly stretch everyday until his knees were almost touching the ground, and at the same time, his ohs improved drastically. Might be something worth checking.
If you're not familiar with the butterfly stretch, it's in one of the performance menu articles by Coach Rut. I think there's 8 stretches actually.