It sounds like a problem with the chamber, either a build up of carbon or possibly a headspace problem. The first thing to try is polishing the chamber. Use a good polishing compound, 320 or 400 grit and a bore mop. Use a 20 gauge mop on a short cleaning rod in a drill. Coat the bore mop with polish and run it back and forth in the chamber for a couple of minutes. Use a medium speed and don't force it into the bore. After polishing clean all of the compound from the chamber and receiver, a spray like "gun scrubber" or gunk-out will work. I would also dis-assemble the bolt and clean it thoroughly. Old grease can act like glue when it heats up from firing the gun.
Try shooting it again. It should fix your problem.
If it doesn't fix the problem you will need to take or send it to a gunsmith to re-cut the chamber. There is a small possibility of just shaving the chamber slightly with a reamer and correcting the problem, but too much will result in the headspace being too big. Normally the breech face and shoulder of the barrel are set back and he chamber re-cut to the proper size.
The compound can be ordered from Brownells. http://www.brownells.com/gunsmith-tools-supplies/metal-prep-coloring/abrasives/lapping-compounds/silicon-carbide-abrasive-compound-prod1142.aspx
Let me know how you make out with it.
If you want to try shaving the chamber slightly I will send you a link for reamer rentals. Sometimes when I have a gun in that is extremely dirty I use a chamber reamer to get the gunk out of the chamber. It cleans it and I know the chamber is the correct size.