are you doing them with an over/underhand grip?
I have scoliosis and due to that have some unsymetrical parts. Try to add more weight and work that side alone during your training.
dumbbell shrugs, force you to use both evenly. and it might help if you stop jerkin off with you right hand and start using ur left haha
If your shoulder blade is winging which it kinda sounds like it is, it could be a nerve problem running through your shoulder that will not allow the muscle in your left shoulder blade to be worked. I sustained an injury lifting and it was because I pinched a nerve and it caused my trap to almost disappear then start to come back. It wasnt until i saw a doctor and physical therapist that they diagnosed what was wrong. On the bad side if this is the case there will be some lifts that will not be able to perform for about 6 months (mostly over the head lifts). The good side is you can still continue working out if you wish. The nerve eventually heals itself. My suggestion would be to see a doctor and ask about a winged scapula. Good luck.
What exercise did they give you in physical therapy? I have a paralyzed thoracic nerve on my left side. Causes winged scapula and has made my left side weaker. I paralyzed it several years ago, back in high school. I've been workingout hard again in the last 9 months, and put more emphasis on the big muscles, and left the little guys behind. Now I paying for.
OP, you got to pay extra special attention to your everyday posture, and form. Keep that shoulder blade in. Chest out, shoulders back.
... It's very possilbe that you "big" trap is trying to do a stabilizing job for your scapula that it just can't do mechanically. ...
I think this is usually the case. Most of us have some degree of asymmetry we have to deal with, and it results in compensatory adjustments. With squats, I have to position 1 foot about 5-10' out more than the other or it doesn't "feel" right. My legs look symmetrical, but the mechanics indicate some small degree of asymmetry. It becomes second nature to deal with these idiosyncrasies after you've lifted enough years.