Hey Mudbug, sorry to hear you have been feeling poorly. Hope you are back to your old self pronto.
Slide not locking back after the last round;
Here is how it works.
The slide must be moved far enough rearward in order for the slide hold open arm to engage the little slot on the left side of the slide. Since the slide is moving very fast, a little built in over travel is designed into the system to allow the arm time to jump up there. Does the arm move up and down smoothly, pistol assembled...you should be able to flick it up and down and it should move freely. Hold the slide all the way to the rear....press the arm down, let it pop back up...repeat thirty times....working smoothly? So, the first test is to fully insert and lock in several unloaded magazines into an unloaded pistol and then retract the slide. Does the arm securely catch the slide every time? If not...stop and figure out why. Is the stop arm damaged at the top, has the slide catch notch become damaged? This of course does not duplicate how fast things must work when firing.
As we all know on the P22 the pull down button engages the stop arm pressing it upward but only after the last round is removed from the magazine. Make sure your magazines are clean.....on the inside and the follower is moving all the way up freely. You can remove the slide, insert an empty magazine and watch the button press the arm up. This should all work very smoothly because when you are firing the slide is moving extremely fast and the stop arm must be positioned immediately in order to catch it.
The most common problem is weak ammo not blowing the slide rearward enough.
Weak ammo, dry, poorly lubricated dirty pistol, dirty magazine - all can cause this problem. Limp wristing, thumb dragging on the slide or resting on the stop arm can also cause it. If everything seems in order I would stretch one of the magazine springs to see if that speeds up the operation of the magazine. If it does....call Ft Smith for some new ones....under warranty. What have I left out?
Did you put in an O ring. That will stop the slide from moving quite as far rearward making all of the above more critical. I've not had any issues here though in 40,000 rounds but it could effect your pistol if the O ring is not lubricated or is too thick. I have seen stop arms become rounded a bit at the top, rear edge which could make it slip off the hardened pin.
The QD recoil spring fits perfectly in a Q slide if you enlarge the guide rod hole. It appears to me to be exactly the same spring used in all earlier pistols. A month ago I fired 2,000 rounds through a drilled out slide with a new QD recoil spring and had no issues. I have it on good authority that Ft Smith has done similar. The length of the recoil spring and washer play no part in this....remember, when the take down lever is pulled down the slide must be pulled even more rearward in order for the grooves to clear the frame rails. So, when the slide is fully pressed rearward against the take down stop....the assembly is still not fully compressed. I'll put some pictures up if it will help. 1917