I've been doing concentration curls for a long time for biceps, and whatever I do I can't seem to lift more than 40lbx8repx3. it's been nine weeks. I've tried resting 2 weeks instead of 1 but that didn't help. I tried lifting 30lb a couple of times after doing my usual 40lbx8repx3 but that didn't help either. I don't know what I should be doing.