I think it's worth adding that the screw that the sear rotates on must never be fully tightened. If it is, the sear won't move freely and that can keep it from entering the full cock notch.
Figuring shdwlkr might not know what the sear is, when the lock is out of the stock you will see an arm sticking out away from the interior of the lock plate.
That is part of the sear and it's sticking out so the trigger blade can push against it to fire the gun.
That sear must be able to freely move against the spring pressure that is trying to shove the nose of the sear into the full and half cock notches in the tumbler.
If it is gummed up with dry oil or as I mentioned, if someone has tightened it up too tight it can't move like it should.
While your in there looking at the interior of the lock, look at the tumbler. That's the thing that rotates with the hammer.
It will have two different notches cut across its face. One has a hook that the sear can slip behind. That is the half cock notch. The other cut is the full cock notch. Look closely at it. It should have a sharp edge where it meets the outside of the tumbler. If it is rounded off, that is the reason your gun won't stay fully cocked.
Let us know what you find.