The Winchester 1200 and 1300 have what Winchester calls
"Speed Pump" actions---a bit different from most. They
use a rotating bolthead, and when you fire them it's almost like
the pump handle is powered back. Very fast action.
Sounds like the action lock on yours is worn if it won't stay locked
on a loaded/chambered round. I sold my last one a couple of
months ago, but IIRC the action lock is interfaced with the firing mechanism,
and it locks the outer sleeve on the bolt in a forward position.
Drop the trigger group out and look for wear----possible on the
black plate with some slots in it on the bottom of the bolt.