I had some issues with mine at first, but I found the culprit to be the magazine retention spring. My problem, discovered by trial and error, was that the factory spring pushed too hard on the magazine, preventing it from moving past the release on the MT bottom metal and fully seating.
I ended up buying an additional factory spring and a mountain tactical spring to try.
Just by eyeballing them, you could see the hole placement was slightly different as compared to how close it was to the bend in the spring - one was closer to the bend, one farther away, and one more or less in between. I measured the distance from the back of the spring to the hole with digital calipers and verified there were differences. I also measured the spring angle with a protractor, and all three were slightly different.
Bear in mind that it's possible to have the opposite occur, wherein the spring hole location is too close to the bend or the angle isn't enough to push the magazine with enough pressure to where it solidly engages the release and seats properly.
The spring that worked in my instance was the one with the least angle which also happened to have the hole approximately equidistant from the bend and the rear edge of the spring. I can't tell you anymore if that was the new factory spring or the MT spring. I'm inclined to say it was the MT spring, though.
In the end I was lucky that it only cost me $40 in springs to get it figured out, although one could argue the time and money I put into solving this problem was more than I should have had to do to make the dang thing work.
That was my experience, and yours may differ.