Going to guess that upper has not been tested/ did run on another lower before hand.

So problem is simply lack of gas to cycle the bolt correctly.  
This could be either leak in the upper gas system or carrier key to top of carrier, gas block passage blocked/not aligned to upper correctly, or just ammo in play is wrong for the set up of the gas system instead.  

I say wrong, since if rig gas action is set up for sonic loads, sub sonic loads will not have enough pressure at the barrel gas port to cycle the bolt correctly, while if set up for sub sonic loads, is going to have the bolt unlocking too quickly, with bore residual pressure too high on the spent case to allow it to be extracted cleanly (pressure welded isntead). And no, if the barrel gas port is wrong sized for the ammo in play, then changing the recoil spring is not going to have enough of an impact to resolve that one.

Buffer and spring are an easy check, and just need to pull all the way back on the charging handle, to make sure that bolt face stops 1/4~1/8" in front of the back edge of the ejection port window, and that during the pull, the tension on the CH stays the same from half way back, to all the way back.

If buffer was too long, receiver extension too short, then bolt face will not make it back this far, and if spring is too long, then going to coil bind out before bolt face gets this far back.

As for spring tension, if super sonic loads are in play, then could be too weak, and the bolt unlocking too fast.  In cases like this, then look at the spent case rim, since bolt unlock too soon as spent case is pressure welded to chamber walls, will cause the extractor to bend the hell out of the rim backwards on the pull.  

If ammo is loaded to too high of working pressure, or problem with leade too short and bullet embeding into rifling on loading, then spent primer is going to be flowed way too close to sides of brass primer rim pocket to show the signs of over pressure their instead.

So really, all points back to the gas system on the upper, with either leaks, or blockages with items not aligned to begin with.  If those can be weeded out as not the problem, then it time to pull the upper apart, to check the gas port size.

And again, with none pistol length gas port location, rig has to be set up for the ammo in play to begin with (sub sonic or sonic ammo).

Ditto, unless the bolt will lock back on the catch after the last round out of the mag, being bolt catch in front of the bolt, and not just catching the bottom of the bolt, then problem is action is still short stroking, or buffer or spring is not allowing to mag to get behind the bolt catch correctly in the first place on a CH pull.


If bolt will lock back with catch in front of bolt, then rifle is stroking correctly, and its the mag that is having problems isntead.  
Hence standard 223 mags have forward inner ribs that help to guide the rounds up cleaning in the mag, and if you are using 223 mag with the 300blk out ammo, then the wider 30cal bullets are going to have problems with the inner ribs and not allow the mag to feed the rounds correctly.  So make sure that the mags in play, are the wider front of mags for the 300blk out ammo, and that the mag catch assembly has been threaded into the release button so the end of the threaded section is flush with  the face of the button.