I had a similar problem with my USP 40 with the Jet funnel system. I placed an HK45c mag release on it. And for some reason the hi cap mags would always fall out after each shot with the new HK45c mag release. I put the old mag release back on and it works fine again. It sounds to me like you may have an issue with the mag release. Have you played with it at all? And is the mag release spring aligned correctly?