I have this problem. Everything gorws pretty well on me. But my arms for years of training have always lacked behind the rest of my body. My arms are getting bigger just not like i want them too. The main problem is that my left bicpet is significantly weaker the my right, i am right handed. When i try and do dumbell curls it seem that i use more shoulder and i can never get a good squeeze out of my left bicpet. Maybe it isn't big enough i have no idea. It seems i just can't make that mind muscle connection like i can with my right bicpet. Also seems like every 3 weeks or so i notice my arms stop growing and i feel the need to change my routine. Any help?