This may sound silly, but give it some thought:
If you are right handed, you are likely favoring your right arm when you handle plates.
Pay attention to which hand you use to pull plates off the rack and bar. Odds are you use your right arm most of the time. Your right arm then gets a better workout than the left in any given training session.
If this is the case, try and force yourself to use the left arm for all plate handling for a couple of months and see if that helps.
Look for a similar situation with the legs. Are you always starting off on the stairs with one leg? Favoring that leg getting up and down? That kind of crap.