sorry to include you ole-cowboy but azzholez like him hit a real nerve with me. Everybody has to start somewhere and answers like his help nobody. My son is 15 years old and I encourage him to seek out help on forums such as this, and to many times he has gotten smartazz answers like rojodiablo's for no good reason. Not everyone is as smart and high and mighty as rojodiablo.ole-cowboy, thanks for your clarification, which rojodiablo should have made without being a smartazz.
Bitter, party of one...... Your table is ready.
I have posted the fix here, in the M&P10 section probably 5 times already. Had you looked for other M&P10 issues in this forum, you would have read my answer.
I am sorry I did not get back to this so fast; been in the Hamptons with the S&W factory workers at their month long getaway, pleading with them to go back to work early for one S&W gun forum member.

Very tough negotiations; a sailboat ride, bribes, threats of being thrown overboard...... it was horrible. I finally relented and gave up my quest when they threatened to take away all my S&W pistols and replace them with Sigmas. I jumped off the boat and swam for shore! (No change in their policy I am sorry to announce. They are still on vacation as we speak, and I am just drying off after my harrowing swim back to shore with all my guns.)
The issue with most AR platform rifles is EXACTLY as was described to you As Ole Cowboy asked me and got an answer in a PM. There is sufficient pressure developed by nearly every ammo made in 7.62 or 308 to cycle the rifle properly. The key to function is timing. There are 3 components to the timing: Gas pressure. BCG assembly weight, and buffer spring pressure.
Spring pressure: Compressive force, etc, whatever we want to call it. More so, it is based on the length of the spring, in relation to the amount of preload. A spring that is a constant rate spring compresses with each coil at the same rate. If the spring is say a 1", 1lb spring, then the spring makes 1lb of pressure for every inch compressed. The preload is how much compression is on said spring, which determines how much pressure is there at rest. If this imaginary spring has 2" of compression, then the initial felt pressure will be 2lb. Spring length is important, not because they collapse so fast (They really don't) but because the buffer tube mated to the spring has a specific length. Get these items mixed up, and you can have issues. THIS is what usually happens with spring related issues. A guy buys a different buffer; they are all carbine length, and the buffer is a carbine length, and now the new better buffer don't work, so the gun is obviously at fault.

Likely, the new buffer is maybe a little shorter or longer, and it can make a difference.
Gas pressure: With sufficient pressure, the gas WILL blow the bolt assembly back. All the way. In fact, it does so with pressure to spare, even from Wolf ammo, and from 55 year old 7.62 NATO ammo from the Korean war. ( Least it did for me last week.) If the bolt is not going back far enough; it manifests itself as casings not being ejected upon firing. If you can imagine looking down from the sky upon yourself as you are shooting, superimpose a clock on you. The barrel points at 12 o'clock. Your rear end is at 6 o'clock. (Is
o'clock an Irish saying??! Always wondered......) Anyway, if your ejected brass is from 2-3:30, you are throwing brass lightly. You might want a bit more gas pressure delivered to the BCG. NOTE: I did not say you need to shoot hotter ammo. NO...... a different gas block may be in order, once you eliminate all other issues. Chamber cleaning is also a culprit. Now, if the brass falls within 3:30 to 5 o'clock, your gas pressure is very good. If the brass flies in back of you from 5 o'clock, or goes out some odd 13 FEET (Yes, saw an AR15 do this last week.
It's a gonna break itself before a too long.... ) A gas bock to lower delivered gas pressure is in order, or the BCG/ buffer is weighted way far off.
Last, most common, and 99.97% likely is your problem. (Actually, it's not your problem. I am not a psychiatrist, so I can't even try to fix that)
I should have said, the M&P10's primary problem, as well as most 308 AR platform rifles which are not working properly.
BCG weight. It's all timing, and weight is part of this. If the BCG is too light, it will fly back very fast; ejecting brass 9feet, slamming into the back of the upper against the buffer, and recoiling forward so quickly that the next round coming up in the magazine can not get pushed into position to be picked up by the bolt as it flies back to the chamber. You get scraped ammo, and mis-feeds. The actual phrase for what we are working on is
Dwell timing: The time in which the bolt slows down rapidly, and actually stops before it changes direction and goes forward under spring pressure. What your rifle needs is to have a slightly heavier BCG, or buffer. This will make it heavier, and slower to accelerate and decelerate. This buys a precious .0003?? second or something of this nature. Juussst enough time where the bolt sits still, allowing the magazine to press the next round up into position properly. It also makes the mass of the BCG assembly a little greater, making the bolt strip rounds more positively, and feed them into battery with a little more authority. You get less misfeeds, and less rounds not fully seating.
THE FIX: Take apart the rifle/ upper from lower. Carefully compress the buffer retainer pin, and take out the buffer assembly. There is a drift pin in the buffer; you need a drift pin punch that fits it. It's likely a 1/16" or 3/32". (Home Depot, Lowes, OSH, True Value, etc....$3.27
Drive out the pin. Inside the buffer, there is some weights. Discs, balls, gel pack, all kinds of different stuff gets used. You want to add some weight in the buffer. Figure the weight of what you remove from the buffer, and if there is a lightweight plastic filler, replace it with a couple fishing weights or ball bearings. Pack them in tight with some foam, paper towels, cotton ball, etc. Press the plastic end cap onto the buffer, and re-set the pin.
Usually, we fix the buffer by adding 1/8oz. to 3/4oz. Usually, it's a 3/8oz fishing weight. Small increments are the way to do this, and test with LOTS of ammo. Mixed magazines, with great ammo like Winchester, Black hills, etc. and follow it up with surplus or cheap ammo like Wolf, Silver bear, etc.
When the weight is right, the rifle will not hiccup, and eat everything you feed it. Thankfully, the weight swap can be done in 1 minute flat once you get the jist of it, and you usually do this, on the bench at the shooting spot so you can test and re-test the results. If you get a lousy result from a change? Change it back. A few split shot weights can make a difference, or a few ball bearings. I like the weights, because they are cheap, the lead is dense and does not fight for space, and I fish a lot...... so I already have them.
I gotta go now. A bunch of irate S&W folks are at my door, demanding I trade my 9504 for a Sigma. Wish me luck as I fight off the horde!!!