FIRST, make sure the mag isn't being forced rearward by the "taper" of the stock mag well. You should be able to put a business card easily on both sides at the same time. If it is binding there, open it a pinch. That will allow the mag to move forward. You should actually move the mag forward a pinch to free up the BHO. If The mag catch is what slides back/forth on the FCG that latches on the back of the mag (mag-catch).
If it isn't binding there, file the front "V" (of the mag) where the mag 'locks' under the trunnion (that's the short tab on the bottom of the rear site). That will let the mag slide forward an RCH and free the BHO to easily work. We are talking thousandths of an inch here, not even hundredths. Proceed cautiously.
Marcus