I have had this happen several times with 2 different mags.  This happened with a new 16in upper (generic Mil-Spec - shop told me it was a Young Mfg) - the other upper (a Stag 20in) did NOT exhibit this behavior.  Less than 200 rds thru this upper.

Any suggestions - or what else should i try.

Thanks,
-Ray

I think Young Mfg. makes bolt carrier groups, not complete uppers. I may be wrong. I'm more interested in what lower you are running? The bolt catch location must be machined precisely and the pin location is critical.

With the lower receiver only and an empty magazine inserted you can examine how the bolt catch mates with the magazine follower to lock the bolt to the rear. With the upper removed and a loaded magazine in place you may be able to identify how it's being levered.

Edited to add: I see you're in California, are you running a bullet button? It may be causing this problem. Custom fitting may be needed to get things operating.

does it have a bad lever if so take it off and the issue should go away .

Thanks for the responses...

(NOTE:   I am the original poster...   I was having problems with my earlier AR15.com account (sandbasser) and had to create a new one...seems fine now).

The Lower is a S&W M&P15.  It worked fine with a Stag 20in upper (and a longer buffer tube etc).  I got the 16in upper and swapped the buffer tube&spring.  I checked out the lower with empty and non-empty mag - i see what is supposed to happen now.

The problem:  first time out to the range shot 120+ rnds and bolt locked back with ammo remaining in mag at least 10 times.  2 separate mags - 1 steel 10rd GI type mag and 1 10-30 PMAG.  

I will try removing my BAD lever to see if that helps.  Will go to the range again this thursday and post results.

Thanks for the responses.

Any more ideas???

-Ray

its the bad lever its not fitted correctly to the paddle on mine I had to grind off the checkering on the paddle so the bad lever would fit correctly right now the rear part of the bad lever is pushing on the upper I bet . its a issue many people have when some paddles have very pronounced checkering .... when fitted correctly you should not see the lip of the rear piece of the bad lever when looking down from the top of it... look at your bad lever from the top then take it off and put the 2 pieces together the way they fit when off is how it should fit when on

bolT

Thanks for the info on the BAD lever.  I finally found some time to go to the range...after removing the BAD lever the problem completely went away.

I opted for removing some material off the back mount of one bad lever that was making contact with the receiver, easily done in just a few minutes and several passes of a file.  Aluminum Black it and back in business.