I assume the gun is a .40? Sounds like it is.
Anyway, I got one of Hennings extended mags for my Elite Match in .40.
The mag tube is a 10mm. This means that the dimensions are maxed out front to back and makes the tolerances very tight in the mag catch area.
The reason for the 10mm tube is that some .40 mags come with a spacer and some do not, whereas 10mm mags come just one way. the 10mm mags also allow you to load out longer (as do the "non-spacer" .40 mags), but I digress.
When I got the mag from Henning, it was tight to push in and would not drop free. Got on the phone to Henning before I started cutting anything. Here is what you want to do;
Take the slide off the gun and remove the grip panels so you can see what you are doing.
Look through the bottom of the mag well and press the mag catch in. You will see a small pice of the mag catch that is not flush with the frame. That tiny piece that is sticking out is what is causing the friction and grabbing on the mag.
If you have a dremmel, you can vey carefully grind away at the corner that sticks out. Go slow! Check for fit frequently. You don't have to remove much - just make the entire mag catch, WHEN DEPRESSED flush with the inside of the frame.
You will find that once you start, the mag will get looser and easier to remove from the gun, until eventually, it will drop free like the rest of your mags.
Hope that helps.
Mick