The technique being used could also be an issue....not sure, of course, if it is or isn't...just throwing out another option to look into. Take a simple dumbell curl for example. If you are lifting the dumbell, which would then include more delt and tri useage, versus actual keeping the elbow in place and allow it to curl up. The function of actualy curling the weight up isolates the muscle group and should prove more effective for you. Also, twist the hand near the top of the curl and squeeze, this help to create more of a peak in the muscle, an issue I fought for a bit. Also, allow the hand to "lag" back a bit...don't curl it forward....which is often done. Curling the hand forward cuases the arms to be lifted slightly more then curling would allow(Arnold said this, I tried it and it workd for me).
Whats the point? Point is, it isn't purely about the weight being lifted. It's about how you lift it. I can't tell you how many guys, younger guys mostly, who lift a bigger amoun of weight then I do, but their technqiue is aweful. Often the technique used is very close, but we do things to make it just a little easier(curling the wrist for example when doing a dumbell curl). Concentrate on technique and allow the weight to come...it will.
Again, just an idea...