When teaching armorer courses, we suggest looking at troubleshooting like this, if the rifle has been working fine and now it doesn't, then ask yourself what has changed.  If you have 20K through the rifle and it is starting to act up, then it is possibly a couple of things like fouling build up, erosion, and possibly worn out spring tensions.  Start with a good cleaning of the inside of the receivers, chamber using a chamber brush and solvent, and scrub the bore using a bore brush, making sure everything is free from any fouling build up.  Field strip the bolt carrier assembly, and clean all the fouling build up from the tail of the bolt, inside of the bolt carrier, clean the firing pin and firing pin channel, clean the extractor, and if possible remove the ejector assembly and clean out any build up in its track.  Once everything is clean, reassemble everything, then lubricate the 7 bolt lugs, and bolt carrier with a good lubricant, my preference is Slip2000 "EWL" Extreme Weapons Lubricant.  Then I would recommend that you change out the action spring (buffer spring) with a new one, as these are usually only good for about 12K, beyond that you are on borrowed time.  Then try shooting the weapon again to see if things have changed.  Try some different ammunition to see if you get different results, and to isolate if its an ammo issue or not.  With that many rounds, you may wish to check the headspace to make sure it is withing spec.

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123