1) Are you absolutely positively sure it’s not your finger bumping the mag release? Sorry, I had to ask.
2) Did you change out the release spring to the heaver version?
3) I found this on HKPro. I don’t know if it will help or not. May be too much sugar for a nickel.
“I did some digging on HKpro as LSP972 suggested, and came up with sufficient clues that I felt I had a handle on what needed to be modified on the HK45 mag release to make it work happily on a USP.
Using a 3/4-inch diameter sanding drum on a Dremel, I carefully recontoured the plastic molding near the projecting metal tab which actually engages the magazine. I removed a bit of material at a time until the contour was similar to the stock USP mag release. (This was non-obvious and I may have never figured out this one on my own.)
The metal tab turned out to be harder, quite literally -- it's made of hardened steel which laughed at my attempts to file it. Lucky for me I have a metalworking shop at home, including an 8-inch pedestal grinder set up to grind hardened steel. Looking at the tab from the side, I needed to grind a bit off of the upper and lower edges of the tip at roughly a 45-60 degree angle so that the tip would engage properly with the polymer magazine catch recess. It took longer to mark the areas to grind away and fire up the grinder than it did to do the actual grinding.
With the excess material removed, the mag release performed brilliantly in competition last Saturday. Mags leapt out of the magwell when I wanted them to, and stayed in securely the rest of the time.”