Bolt over base malfunction.  Either your slide needs to slow down or your magazine needs to speed up. I would suggest a stronger recoil spring next.

Totally agree with Sqlbullet, regarding the slide outrunning the magazine. I'd also take a look at your barrel fit. Recoil spring and barrel fit will determine the lock time on an M&P. Take a look at this if you're not familiar on proper barrel fit. https://www.youtube.com/watch?v=QqCu1jnVwZ4

Good luck with it. Sorry you're having such a problem with yours. My 4" M&P 10 runs like a champ. Hopefully yours will too when you get it figured out.

 The pic Weldmeister posted above with the old and new mag spring and follower...  Notice the difference in the spring length.  I have a 4.6" model I bought on 12/16/21 (manf date on the box is 12/7/21) so its one of the first off the line models.  Have not has any issues with it from shooting soft S&B FMJ, (hot) Hornady Custom XTPs, to nuclear Federal HSTs.
  It doesn't really eject the brass to the other side of the county like my custom 1911 10mm does either.  Even with the hot HSTs rounds.  You need to bring the bushhog to retrieve the ejected brass from my 1911.  And make sure nobody is standing to your right side also even with a FFP stop, increased main spring, and 22# recoil spring.

Both factory mags that came with the 4.6" gun in December have mag springs that measure 6" long brand new the day I bought it.  I always measure mag spring length and record it for my own personal database of info.

Approximetly 1 month ago my local shop got in the first batch of 10mm mags for sale and I recieved a text saying they were in.  I bought 2 of them that day and tore them apart and measured them that day.  The S&W brand replacement mags had springs in them that measured 7" long now!!  That is 1" longer that the mag springs that came with the pistol.  Wire diameter and coil count remain the same.
  I bought some Wolff 45 springs to measure them for reference and they measure 6" OAL length.  Wire diameter tha same and 1 less large coil on bottom.  So even putting 45 springs in there probably isn't going to gain you anything.

  But in any event even though you can feel a definite difference when pushing down on the follower between the old 6" and new 7" long springs, I have not had any issues with either sets of mags yet in my gun.  So jsut an FYI:

Steve

So I found this today on YouTube about the possible issues I am having with this gun, and as most of you have said, it is related to weak magazine springs.  Worth the watch if you are having this problem.  I ordered a set of Springer Precision Magazine Springs today.  Will report. Greybreard, I did check out the barrel fit as per the information you supplied.  No problems there.  This pistol is not going to win.  It will function, just gotta get it sorted out.


https://www.youtube.com/watch?v=DXb8_MNLvPk   

Just to circle back around to this, stretching the mag springs on mine resolved my issues. I haven't had a failure in a couple hundred rounds with either mag that it came with.

Maybe we will get a 10mm with the new metal frame in the future. Probably not but I can wish.

 So... Got the Apex extractor and installed it today. Took the M&P out to the range and tried it out... I gotta say... This part has cured my problem! I only ran 100 rounds through it, but had zero problems. And this was with full power, 200gr bullet at 1200 fps loads. I will keep monitoring it, but I have to say thanks to Apex for helping me get this thing reliable. I played with magazine springs, recoil springs and everything else, but before I installed the Apex extractor, I couldn't get through one magazine without failures to feed, failures to eject and loaded rounds puking out of the ejection port. Today's test was with the stock magazine springs, and the 24lb recoil spring.

This fix was suggested on a FB  M&P page.   The explanation I got I will paste below from Scott Folk at Apex:

Scott Folk:
  "Most people don't realize that the extractor in a pistol is integral to the feeding process. The stripper rail picks up the top edge of the brass to advance it forward out of the mag. As the bullet rides up the feed ramp, the rim of the cartridge slides up against the breech face, but not flat against, the bottom edge is away from the breech while the top is against the breech. The extractor has to be cammed out by the case rim against spring tension. There is a necessary feeding cut on the bottom of the extractor that helps cam the extractor out, we call it the feeding cut. What we found on the m&p extractors is that the feeding cut angle is not optimal and often creates too much tension against the case. In the case of a 10mm, due to the radius of the cartridge and speed of feeding, often the case jams against the entry cut causing a feeding malfunction, or the extractor can't cam out which causes the brass to pop off the breech face.
At Apex, we happened to receive 2 10mm that wouldn't feed for Jack. I did some experimenting and determined that the pistol would feed cleanly without an extractor, which meant that the extractor was the cause. We did some magic Apex Voodoo that we do and we're able to create optimized feeding geometry.
When the original m&p launched, mine had terrible extraction, it was so bad that in a uspsa match, my long rotating transition was fast enough that I caught a piece of brass on top of my slide between shots. This caused us to redesign the original m&p extractor and call it our Failure Resistant Extractor or FRE. We applied a similar technique to the Glock gen 3 extractors and were able to drastically improve the ejection pattern of the G3 platform.
All this is to say that we've been doing this a long time and we've gotten pretty good at fixing feeding and extraction issues at Apex."