Greetings Bootstrap, and welcome aboard.
The first red flare I see is the loose sear pin, was it partially removed when you were shooting?
Under normal circumstances, if it just quit returning all of a sudden, then it's probably a bad firing pin return spring. Other causes could be gunk in the action, lack of proper lubrication, a worn firing pin or worn pin hole in the action. Have a look at the breech face, are the firing pin holes round or worn to an oval? If worn to an oval or ragged (also caused by primer blow-by with reloads), it will require some serious smithing.