Not sure if this is the same issue, but in the past year on the range I've seen several instances in which students had new production Glock mags with clunky feeding or failures to feed. Each time I have traced the problem to the plastic tab on the inside front of the mag (about 3/4" down), where the plastic inserts into the steel mag body -- i.e. a bit too much plastic there, interfering with the follower (or perhaps grabbing at bullet noses). A few strokes with a file, or simply scraping away the excess plastic with the tip of a blade, has cured the problem in short order.
I've seen this 3-4 times in the past year, to the point that I now probe this spot with a punch on any new mags I buy.