For some reason, I'm having a lot of trouble training my biceps. I do the usual lineup; standing straightbar, preacher, dumbell and concentration curls, but for some reason my biceps never seem to get stronger. At first I thought it was my form, but I looked it up and I'm doing everything right. I'm using the heaviest weight I can while keeping my form good. It's not a matter of protein intake (I take in 1 gram/lb of bodyweight every day). They grow bigger, but not stronger. Anyone ever have this problem, or one similar?