I have the same problem with my PC Shield 9. The 8rd mag with Hyve extension worked fine in my standard shield 9 before I sold it. Same mag, with Hyve, wont seat in my PC Shield, no matter how hard I push. The catch hole in the mag body just doesn't reach the mag catch. Looking at a standard mag (with Magguts +2) side by side against the 8rd with Hyve, there is a visible difference in the length between the top of the base plate and the bottom of the mag catch hole. This is the cause of the problem.

Removing material from the top of the Hyve, as stated above, will likely solve this. Not sure how I feel about sanding/grinding it though, due to removing the finish.

I plan to call Hyve today for their input.FWIW, my brother also has the Hyve on his shield, and it works great. I'm going to test his in my shield, and see how it works. If it does, I plan to disassemble both mags, and measure them to see if there is any variance in length between the bottom of the mag and the mag catch hole. I'm thinking that the manufacturing tolerances/differences may be present in the mag body, in addition to the frame of the guns. It may be worth looking into trying the Hyve on a different magazine body, but those aren't cheap either...