+1 to those that said dirty chamber.
I use a .25 brass brush and Gunzilla to clean my .22 chambers. I bend the brush so it's no longer than a .22 (to make sure I don't screw up the throat) and that, and a little solvent/lubricant takes care of the problem. When I get done with that, I pull a patch on a Patchwork from the muzzle end so as not to spread the gunk down the barrel.
BTW, try this, and when it works, find a new gunsmith - it should have been the first thing he or she tried.
When you have a clean chamber, you should be able to drop a round into the chamber, have it seat all the way to the rim, and be rewarded with a most satisfying "plop" sound. Invert the barrel and the round should fall out on its own. If this doesn't happen and you're positive the chamber is clean, it's possible that your breech had been peened by someone else dry firing, in which case a chamber reamer HAND POWERED (that is, twirled by one's unaided fingers) may be cautiously employed to restore the breech. Chamber reamers cost somewhere around 'more than I'm willing to pay', so that's a job for a gunsmith. Fortunately, there's a 'smith in my club, so I'm covered.
Let us know how it all turns out.
All the best,