Full range of motion? It's the biggest mistake I see with guys in the gym. They think they are loading the muscle up by hitting them with 500+ lb shrugs, their problem is, their shrugs aren't really shrugs. If you want to see some growth, try this, try a full range shrug, meaning, take the movement up to the point where your traps are going to hit your ears. Then hold the bar for 1 to 2 seconds, and let the bar down slowly.
Too many times I see these guys throwing 6 and 7 plates on a side, then then basically rolling their shoulders. A proper shrug should be a straight up and down movement as opposed to a should roll.
*shrugs* Another way to get your traps growing is to alternate the location of the bar, (front or behind) standing and seated. Every few weeks I will get a shrug barput it under a bench and shrug with the bar under my thighs. There are many different methods, but the biggest thing I can suggest is to make sure you hit the full range of motion, and not to worry, how many plates you have on the bar, as long as you get the burn, who cares?