In this case, SOME Thermolds are not finish trimmed in back. There is a U shaped cut where the finger of the bolt stop contacts the rear of the mag follower. The bolt stop bottoms out high in the bottom of the U and the mag won't seat. With that area properly trimmed (compare to the rectangular cut of a USGI mag) the bolt stop will bottom out correctly and the mag will seat. A Dremel is the tool to use here to turn a problem mag into a perfectly workable mag.
Pictures proved upon request.
The other problem, there is an angled rib on the Thermolds that contacts the beveled area of the mag well on bottom. There was no requirement for any special angle on "mil-spec" receivers as the USGI mags don't have this rib, only the Thermolds, which are not US mil-spec. Even the Canadians don't use them now. So, this rib interferes with some receivers, and trimming it will fix that problem.
I don't take a Dremel and just grinding for no reason. It is aways easy to remove material, but a real problem to put it back on later. When I troubleshoot, I like to really spend time on getting to the bottom of the real problem.