If you have a "spare" sear, swap it in. If the "long arm" of the original sear is distorted, (bent), it may well be engaging the very end of the half-cock notch and this not locking up the bolt and trigger movement.
Swapping in an "unmolested" one is pretty easy, after removing fore-end first. You then have to attach the trigger-guard assembly in its correct location (sans wood, of course) and see what is happening.
Please keep us posted.