It's a pretty common problem man. Make sure your form in the same with with both arms. With one handed exercises, like db curls, the motion may vary in each arm resulting in a different shape of the muscle. Also, if alternating left to right such as with db curls, tricep extensions, ect, do the weaker/smaller side first so you are less fatigued and can focus more on it. I do this now even with db curls where a alternate sides each rep. I always start the first rep with my left arm because it's lagging behind the right.