With the stock off, open the action all the way and observe both hammers. If the sear doesn’t set, reach in with a prying tool and manual cock the offending hammer a bit more, if that causes it to set, you have most likely a wear issue with either the cocking lever, or the metal in the forend that it acts against. If that doesn’t make the sear set, the gun will need further disassembly to find the reason. If that’s the case, STOP SHOOTING IT until it is repaired. I’ve seen a piece of shot smashed into a hammer not h that caused a similar issue but also sometimes would fire when the action was closed (not good).