Before 1998, all of the models used the old style 3 piece sear/bolt release. It is not uncommon for the lever to work loose from the sear, but it will still remain in the right position as long as the sear pivot pin is in place. What you are experiencing is the pin is sliding to one side after numerous firings and slides out of the hole in the thumb lever, disengaging it as a lever.
Simple fix: reverse the pin so that the head is on the same side as the lever.
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