First item to check, is the top of bolt stop slightly up to far, and slowing the bolt back down on the way forward.
To check for this, ride the charging handle back forward and see if you can feel the carrier binding on the way forward over the catch.

From here with B/C out, make sure that the Forward assist pawl is retracting all the way back, and not still protruding to catch the side of the carrier.

With mag body alone lock in the mag well, make sure that the B/C is not biding on the back U slot of the mag as the B/C goes forward.

Pull the buffer and recoil spring out of the tube, use a file to clean up any end of spring coil burs that may be scrapping down the inside of the tube, and take the file to the buffer bumper roll pin to make sure both end of the roll pin are flush with the surface of the buffer.

Could be that the top of hammer is catching the firing pin stop collar enough to slow the bolt down on forward stroke.
Here, check the top of hammer for marks, the firing pin retainer bent, and/or firing pin proud of the edge of ramp when pushed all the way forward.


From here, pull the bolt off the carrier and with just the carrier and key, dry fit it into the upper to make sure that the carrier is not binding in the upper, and when the front of carrier gets about a inch off the face of the barrel extension, gas tube enters the key cleanly, allows the carrier face to touch the barrel extension face without a lot of needed pressure, and when you turn the muzzle up, carrier falls back down off the key via it own weight.


Next, is the extractor and the extractor spring.  If the extractor spring is extra tension, make sure there is not O ring around the spring.  It will cause too much tension, and the extractor will not be able to climb over the case rim on feed.

On the extractor still in the bolt, look at the top of beveled sides of the extractor claw, and they should be flush with the ID of the rim surface.  Hence on feed, rim has to be able to work down the beveled area of the extractor to open it up so the rim can slip it open, and if the extractor is too closed, rim will not be able to open it.

This is fine,


While something with the extractor top of entrance bevel more inward, could end up with the rim of the round not able to pop the extractor open on feed isntead.




Next, hook at spent case on the extractor, and cam the ejector into it bolt face channel a few times to make sure that the ejector is not binding in the bolt face channel.

Last one to check, is to pull the B/C and just look at the top and bottom of barrel extension feed lips, and make sure that the top one is dead center of the gas tube channel, while bottom one is dead center of center line of lower front lug.  Hence if barrel extension is canted in the upper, can cause the bolt lugs to crash into the barrel extension lugs on feed.

To add, if you are shooting reloads, put them through an ammo gauge to check the rounds.  Hence cases may not be fully sized to start with, or could be too much bullet crimp pressure to bulge the shoulder during bullet crimping, which will have the bolt lugs binding up just enough that the bolt lugs can not get inside the barrel extension void to turn on the last of the lock up.

This...

Over the years on single stage press set ups, and even progressive machines, have seen it a few time where even if the die is tight against the shell holder or plate with sizing tension pressure on the machine so the two are touching under full sizing pressure between the two, case is not full sized and had to face a few sizing dies a few thousands to allow the combo to size the case correctly.
Note, Dillon sizers are already shorter than needed at the face, and why when you are setting up a sizer in a Dillon for bottle neck cases, really need to see a case/ammo gauge to dial in the sizer to size the brass correctly.

What happens more often, someone will eyeball the two without sizing tension when installing the sizer, and when actual sizing where you have pressure between the two, ends up with a nasty gap between the two that does not allow the sizer to full size the case/die to come all the way down to kiss the shell holder/plate instead.

Bolt is not caming over with bolt lugs in the barrel extension void, so could be extractor entry angle ramp is could use some polishing to angle it more outwards on the radius of the bolt recess channel to allow the case rim to open the extractor on feed better, could be extra tension extractor spring or O ring on the spring that has the tension too great for the case rim to open the extractor to allow the rim to seat on the extractor rim relief cut, could be once in a blue moon binding of the ejector to not allow the case to seat on the bolt face.

Or, could just be on the tighter head space side, and once in a while live round in not sized down enough to allow the bolt lugs to get into the barrel extension to cam over instead.

Has the bolt and barrel extension been head spaced checked, and has the ammo been run through an ammo check gauges?