Well in the last couple weeks my M&P9L started to dump half fired magazines half way through Steel Challenge strings. At first I though it was just me getting sloppy and not seating the magazines. Keep in mind I have been shooting Steel weekly since the beginning of April until a couple hickups with the mags in the last couple weeks, then in the last two stages of a local match it started dumping them almost every string. Took everything apart pulled out the magazine release, cleaned all the mags, everything put back together and still happening. Go on the horn with S&W customer service told him what was going on and without hesitation he said he has a new on in the mail for me to install. Great customer service, but definitely a known issue.
Just provided as info, was supposedly an earlier issue that was resolved but still cropping up in some instances.
Good luck