My 15 year old son is having trouble getting his left arm to lockout on the jerk portion of his CJ. At lighter weight there's no trouble, but when he gets up to or close to his max, his left elbow sometimes breaks slightly before locking completely out. There's no problem with his right arm locking out even with heavier weight. I was wondering if this is a strength issue with his left arm or a technique problem. I appreciate any and all suggestion to correct this issue.