It seems you've done what you can to try to narrow down the possible issue you are experiencing with your air rifle. Respectfully, I don't know how advanced your skills are, but what many fail to implement before taking a shot is finding their body's natural point of aim (or NPA). Finding NPA is crucial if you want to hit with precision, let alone just trying to get good shot groups. When you neglect NPA, what happens is that your rifle never really defaults back to pointing or resting in its neutral position because even the slightest tension or pressure imparted on it is never the same every single time you line up a shot. And, so, when shooting, just because the crosshairs on your scope is locked on your aim point it will not necessarily mean that the pellet is going to land exactly on that spot. Sure, maybe you'll hit your aim point once or a few times, but almost always it will never be every single time. It's because at those times when you get a flier, following a shot your rifle will point in a different direction than the one it initially was in.
There are many explanations on how to find NPA online. But you must know that to get to it your entire body needs to be fully relaxed. If it means that to be so requires resting the full weight of your firing hand on the stock, so be it. Know that when you are fully relaxed--as if you are dead--when in contact with your rifle, you as well as your gun both “agree” to a single point of aim, not two. This is why there is the adage "you must be one with the gun."
Good luck!
