Since you're an MG guy, there's good chance you haven't cleaned it yet...it could simply be due to the accumulation of fouling over the tighter tolerances of new barrel chamber from running FA, enough so that is preventing it running SA correctly.
Did you check appearance of the casings, particularly the stuck ones (they cay indicate if surface irregularities inside the chamber)? Have you gotten good look at the chamber? It could be needing light sanding and/or a good polishing job depending what you see.
Remember to inspect your spare barrels too. If headspace is excessively tight, might need to have them reamed.