Came across this old thread as I was having the same issue with a brand new T8.1 where, for no explicable reason, the magazine would suddenly not fully engage with the magazine release.
Therefore I'm posting what I discovered in case anyone else is having the same issues.
The problem with my T8.1 was that the mag ball retainer pin, and mag ball retainer spring located just above the follower suddenly became too stiff on BOTH magazines at the same time.
I'm not sure but I think this bar pushes up against the ammo release fork as you insert the mag and then snaps into place. However, when it is not lubed properly it won't move and snap in place.
The solution is to run it wet and make sure the teeny spring, pin, and the guide in which the pin slides is lubed.
Now, I should have used a silicon based hobby lube (like those used for RC cars) because petroleum products are very bad for rubber o-rings, seals, and makes plastics fragile. But I only had some Corrosion X CLP gun lube on hand (since I shoot real steel and bought the T8.1 to be able to practice indoors). I sprayed the mag ball retainer pin and spring, loosened it up by moving the pin back and forth a few times and the mag now snaps in place without a problem.
Hope this somes in handy to the next guy looking for the solution to the problem of why the T8.1 mag suddenly doesn't snap into the mag well.