AyA 20 ga 4/53 boxlock 1996 vintage with little use. With Remington gun club shells (1200 fps - 7/8 oz the safety returns to safe position when right barrel is fired.
When using my reloads 13 gr 20/28 powder 3/4 oz shot (1155 fps with about 6400 psi) the gun functions correctly.
What might be the problem? Is it very serious and is it difficult to repair? Guns weighs 5 lbs 12 oz
Thanks in advance for your input

Can you tell that the Remington shells have more recoil? It looks, from the load, that they may. Sometimes that is the issue. Easily fixed by putting a little more tension on the safety slide spring. Does the safety slide move really easily?

SRH

Stan it is lighter than my other doubles, but it definitely clicks off . There is more felt recoil from the Remington shells. I would only use that loading when hunting wild quail in Texas
Just had opposite problem with WJ Jeffrey 16 ga double and gunsmith had to lessen tension on safety spring because safety was to stiff

My wife had this issue with her 16 gauge Ugartechea boxlock with both Remington and Herters factory ammo. It was a simple and inexpensive "bend the safety spring" fix for one of our gunsmiths.

Lighter will definitely give more "kick". The increased payload plus the increase in velocity could combine with that to cause it. Not saying definitively that is the problem, but it's the first place I'd go. Sometimes it is not the inertia of the slide that actually causes it, but an inadvertent movement of the thumb under recoil, that moves it rearward as the gun "returns to battery", after the actual rearward recoil. This seems to happen more with straight gripped guns than pistol gripped ones, in my experience. IMO, the hand is more likely to slip forward under recoil, with a straight grip, thus setting you up for the thumb to catch the slide and pull it rearwards as the gun returns to battery.

You might try adjusting your grip for a few shots to be absolutely certain that is not happening, before pulling the buttstock.

SRH