Welcome aboard.
A common error with SIGs (P22X series) is holding the gun in such a way that the shooting hand's thumb rests or touches the slide stop during shooting with enough pressure that it prevents the stop from activating when the mag goes empty. I've done it myself some times early on my first SIG (P226).
If this is not the case and the stop does indeed not go up when the mag is empty and it is not the mag, it could be that the slide stop spring (which applies a slight downward pressure on the stop to prevent it from accidentally flipping up during shooting) is bent a bit too far and applies more pressure on the stop than normal. To inspect and possibly modify the slide stop spring, (I would suggest that you first make sure that your thumb is not holding the stop down) it is easy to take the gun apart to examine the spring.
To remove the spring/locking block insert :
Disassemble the gun normally and de-cock.
[Frame] Rotate the Takedown Lever to 12:00. Direction of rotation is not important.
Pull/wiggle out the Takedown Lever. When the Lever is pulled mostly out but stops at the left side of the frame, rotate the lever 90° forward, i.e., from 12:00 to 9:00, and pull it the rest of the way out.
Note the orientation of the Trigger (hinge)/Locking Insert Pin. For the P-220, the end of the pins should have serrated grooves. The lines should be going front to back level.
Lift up the front and pull out the Locking Block Insert and Slide Latch (Stop) Spring. Note that the orientation of the Pin serrations, they should be 20°-45° depending on how careful and fast you are. The Slide Latch Spring is what is you should look at (and possibly modify if needed). Note that the Spring is merely attached to the Block by hooking into a hole on the side.
You can, at this time, also remove the Pin and remove the Slide Latch itself if you want to. I usually remove the Pin using the Spring to push it out a little to pull it out with my fingers. Note, however, that the PIN has two notches on it. These notches need to be in the front position when the Pin is re-inserted (serrations at 20°-45°). The notches on the Pin allow clearance for the mating notches on the Insert the Pin goes through.
When you re-install the Insert (with Spring) into the Frame, the Pin serrations must be slightly inclined 20°-45° and the Slide Latch positioned against the frame. Otherwise, the Block will not go back in correctly. If the Latch is not against the frame, the Block will not go in. If the notches between the Pin and the Block do not mate correctly, the Takedown Lever holes will not line up. The Pin can be rotated by fingers at this time to allow the notches to line up. That is the reason for the serrations, so that the fingers can grasp the ends of the Pin to rotate.
If you look into the Takedown Lever holes, you will note that the Slide Latch Spring protrudes into the hole a little. The right end of the Lever pin has a notch on its corner. Insert the Lever positioned 12:00 so that the Spring will be in the notched portion of the Lever pin end. If you look through the Lever holes from the right, you can see the Spring go into the notch.
Rotate the Lever 90° either direction to cam the Spring out of the way and push the Lever in.
With the Lever mostly in, it should stop as the end of the Lever pin reaches the right inside on the frame. The Spring needs to be cammed out of the way again to allow the Lever to be fully inserted. Note also that there is a small notch close to the arm of the Leveer.
With the Lever as far as it will go, rotate the Lever to the forward poistion counterclockwise. The Lever will not cam the Spring out of the way going Clockwise.
Push the rest of the way in.
Rotate Lever to the 6:00 position.
Re-assemble as normal.
The small notch near the arm of the Lever allows the Spring to extend into the Lever pin. This prevents the Lever from walking out under normal recoil/operation.
This is for the older flatter Lever P-22X. Not entirely sure about the current non-flat Levered SIGs.
