I had a similar issue with my S&W Military and Police Compact .45acp.
The 8rd PVD magazine could only load 7 .45acp rounds(Critical Duty 220gr).
I took the M&P magazine apart, ran a cleaning rag with LPX,
www.Mpro7.com over the parts(including the spring). It took some effort but but the M&P magazine loaded 8/eight .45acp rounds.
Im going to keep the M&P mag fully loaded for a few weeks to get the magazine used to the full 8 .45acp rounds.
The other 8 round PVD treated factory magazine worked fine with 0 issues.
Clyde