I raced today at a pretty demanding circuit course that required a full range of gears, from my 34x25 to my 48x13 (junior gear restrictions mandate that my top end gear is no more than that) and there was a fairly rough section of potholes and stuff during which I must have messed something up, because the rest of the race my downshifting was sketchy, it wouldn't shift into my 13, and it couldn't stay in either the 25 or 23 without skipping bad. I checked the hanger, dropout and tension and it all seemed okay...what the heck could have happened?
Check your cables, I believe its to mucked out with crud... It upshifts alright, but does not downshift properly ?
Check for kinks in the shifter cables, it might be the cause too.