If you've been fitted for just riding on the hoods, your setup may be slightly off too (saddle tilt, set back, height, etc.). On my last road bike, I was set up by my buddy who owned the shop. He smacked me when he said "get in a comfortable riding position" and I put my hands on the hoods. He knew how I rode, and expected me to get set up for maximum efficiency while using the drops. I complied.

As with others, gradually increase time in drops and lose weight if you've got any flab around the middle.