I took a mt. bike clinic this spring and had some issues turning. Basically, I had crashed earlier in the day and had tweaked my neck. I could turn it to the right, but not to the left. Later in the day, we were riding some switchbacks and I was turning right just fine, but couldn't go left to save my life. I have to admit, it was pretty funny -- I'd make the right turn, get to the next switchback and have to stop, lift my bike, and turn and then get going again. Apparently, you have to be able to turn your head in the direction you're turning or you can't make the turn. Is this perhaps your problem?

Susie