Funny thing that happened to me that is similar to your issue, it could be that your SA ammo is very dirty. I had just put new springs ang GF's in all of my 30 round mags, and the kit came with a few hundred rounds of SA ammo. Before I shot the SA ammo I was using my XM193 and Q3131A with no problems. Then I was down to the SA ammo. I kept having feeding and chambering malfunctions. I thought it was the mags, but they worked with the other ammo all day. Then my buddy with his brand new Bushy dissipator tried the SA ammo in his AR and mags, same problems. Sometimes the rifle would fire a round and the second round would not get stripped from the mag, other times it wouldnt chamber without a little help from the FA. We both had a large number of FTF/FTC malfunctions, but only with the SA ammo. Although ammo is not a normal cause for these types of malfunctions, in my case there was no doubt as to the cause, and It could be the same problem you are having, I would try the same thing with different ammo, if the bolt locks without the FA then clean or sh*tcan the ammo.