When most lifters think of their traps, they visualize only the small portion of the upper traps they see from the front. But the trapezius is a big muscle, originating at the base of the skull, running down to the upper and mid-back, and running width-wise covering both scapulas. If all you do is standing vertical shrugs, you're "missing" much of the muscle.
Try chest-supported incline shrugs at various angles. Each angle will target a particular portion of the traps. Use a steep angle to hit the area about halfway between the upper and mid-traps. Drop the bench a bit lower (about 30 degrees) and you'll target the mid-traps, which is where most people are lacking.
Just lay chest down on an incline bench. Shrug up, hold, and squeeze at the top. You can use either a neutral grip (palms facing each other) or a pronated grip, as you would on a standard bench press.