Although Jammin's fit correctly in the beginning, as she has muscled up and filled out it has begun to bunch exactly as you describe, although when pulled back the loop is still in the right place.
My harness is custom made X-back from Alpine Outfitters and it has been amazing!! It fits me just right and I love to wear it. I always help mommy put it on when we use it... head, right paw, left paw, lets go!!!!! Oh what fun!!!