It may be that you are simply putting to much stress on your arms. For me it is somewhere in the middle. I have decent def in my right arm but not as much in my left. However, my left arm is bigger.
Try using a preacher bench. It will make you use a very strict form which could be a part in your problem. Hammer curls are great but they will just add to the size of your arm. However, they do give a good separation between bi and tri.
Also, put alot of emphisis on the tri's, they can be a very beautiful muscle when they develope correctly. I myself am trying hard to get that deep tri horseshoe look.
I don't know if any of this rambeling helps, but God Luck!!!