How heavy are your shrugs? I have found that if you're not struggling to do 10 reps with the weight, it's not enough to make those traps grow. If you have access to a smith machine, this makes it much easier as you won't have to lift the weight all the way from the ground and have safety stops in case of fatigue. It makes baby jesus cry every time I see kids tossing 135lbs up on the smith machine to do this and pop out 25 reps, which is a total waste in my opinion. (Unless you have issues with 135 for 10 reps x 4 sets)
My .02c