I have owned a couple of Hatsan Twangers, & could not shoot a tight group. One day, I dropped the artillery hold & held it like I was shooting a high powered rifle. That did the trick !
When a specific gun begins to wander POI the first (and easiest) thing I do is clean the barrel and check for seal leaks and loose screws.. Next would be to check the scope.A chrono is one of the best tools to diagnose a sudden change in accuracy and performance.
Do you practice the quattro trigger pull? Slightly upwards at about a 30 degree angle.-Y
As othesr have said, CLEAN THE BARREL. That and stock screws should be the first check. Do not over tighten your stock screws.Try shooting the Box Test to see if the scope turrets are working properly. Did you crank the elevation way up? Perhaps dismount the scope, set it to optically centered by the mirror method.Remount scope, torque rings to spec. See if that helps. Do you practice the quattro trigger pull? Slightly upwards at about a 30 degree angle.-Y