Seems to me that if the firing pin is not putting a good dimple in the primer, it has to be (1) weak spring, or (2) broken pin, or (3) crud/rust in the pin housing, with (3) being most likely.
One other possibility is that the action has loosened up and there is too much space between the breech face and the primer. Have you tried backing the breech plug out a little so that when you close the action it's snug against the primer?
If the pin is putting a good dimple in the primer and the primer is not going off, you've got contaminated primers.