It'll be difficult to get serious help without video footage (I don't own a video camera), but I'll do my best to explain the problem anyways.
It was pointed out to me the other day that when I break parallel on my full squats my upper back rounds slightly. I grabbed a broomstick and checked out myself in front of a mirror when I got home, and the guy was completely right. I think I need to keep my core tighter and pull back a bit more, but when I do this I can't break parallel.
Is this likely a flexibility issue or is it something else? And, if it is a flexibility issue, what would be the best way to fix it (i.e. what exercises)?