Remove the buttstock first, then remove the trigger plate screw and slide the trigger group back off the receiver.. Then, while putting backward pressure on the forearm, use a rubber mallet and give the foremost end of the forearm a couple of gentle taps.. Shouldn't take more than 1-2 and the shell should come loose.
It's sticking in there because there's fouling and/or rust in the chamber.. Once the shell's removed, ensure you remove all fouling/rust from the chamber and reassemble..
Check the primer on that live shell to see how the firing pin strike looks. If it's shallow (and if all the former spent shells show shallow dents) it's possible you may need to send it to a 'smith' for examination and corrections..
