If the lock works when its out of the gun, then the problem is almost certainly that something is binding in the stock mortise. Don't be cranking those lockbolts down as hard as you can. You can collaps the thin edges of the wood in the lock mortise, and that will bring parts of the lock in contact with the bottoms of the mortise, where the wood will bind the parts from moving properly. Put some lamp black, or lipstick on the inside edge o the stirrup, and tumbler, and " fly ", and install the lock as you have done. Cock and lower the hammer a couple of times. If its binding something, the marking dye will transfer the part that is being bound, to the wood in the mortise. Its a simply matter to remove the extra wood with a chisel, or even a pocket knife. I like to seal the wood in the mortise by applying a very thin coat of stock finish. Anything to keep moisture from the air, rain, or from cleaning the gun into the wood and causing it to swell. If you store you gun standing on its buttplate, you lock may have oil, or water that has seeped into it and caused the wood to swell and bind your lock. If so, you will have to wipe the mortise with alcohol to break down the oil, and evaporate it before sealing it with stock finish.