Spring piston rifle scope not holding zero

Having trouble getting my rifle to hold zero.

Equipment:
Gamo Varmint .22 33mmOD/ 29mmID cylinder spring piston break barrel
Offset one piece scope rings
UTG bug buster 3-9x32, rear shimmed
H&N Baracuda 5.52mm 18.13gr

Shooting position:
Seated bench
Block for forward rifle support; no rear bag

Summary:
I’m looking for any advice or thoughts on zeroing my scope and getting it to hold zero. I got my air rifle about a month ago for pesting needs. Shot about 300-400 pellets through it. So far I have tried 3 different pellets and so far it seems to like barracuda 18s. Once I zero my rifle at 20yards, it can reliably shoot 1/2” groups at my shooting table and often drill the same hole. I’ve become pretty comfortable with my artillery hold although the heavy trigger sometimes makes for a tough shot particularly at ~30yards. Approximately <1” grouping.

Every so often, my zero will shift slightly when I check and shoot targets. One time I realized one of scope ring screws was loose so I checked and retightened them and then re-zeroed. Then another day it was off slightly again although holding a decent group. POI can be off both vertically and/or horizontally by 1” to 1-1/2” at 20 yards. It’s getting frustrating not being able to depend on my accuracy when I need the shot to count for pesting. Any advice?

-Should I loctite my scope screws?
-Is my scope broken? I have been trying to make sure the rings and scope are secured but also being aware of not crushing the scope. The rifle has quite a bit of kick.
-Is the accuracy shifting a sign that the barrel is still breaking in?

Really sorry for the long post. Tried to provide as much relevant detail.
 
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On harder kicking, springer, a one-piece mount is better; they also come with 10 or 20-degree built-in them for barrel droop. That's the way to go.
I found myself in a weird predicament during scope mounting. And just a reminder, this is my first air rifle venture and I was on a tight budget so lots of learning along the way. The eye relief on the UTG scope required me to choke up on the stock to get closer to the scope than I anticipated so I got a single piece offset mount that allowed me to push the scope further back. It was then during zeroing that I realized I ran out of elevation adjustment and learned about barrel droop. So in an ideal world, I would have loved to know all these details beforehand to 1- decide on a different scope or 2- find an angled, offset scope mount. But hey, atleast I got something to shoot with and I finally have a pest free backyard! (currently anyway)
 
Dont that gamo come with a gamo package scope on it out of the box ?
It does but no AO or magnification. After reading and learning more, I realized that back in the day when my best bud and I would plink with his old Gamo, the reason we probably couldn’t get it zeroed at all was the scope probably came broken. So I budgeted a scope when I purchased my rifle. If the utg is broken, hopefully I can send it to back to utg for their lifetime warranty and use the Gamo scope as a backup.
 
Try using 2 sets of rings.
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