Have a short list of non-drastic (can reverse them) things to try.
#1. Clean the barrel. Probably did at least once, but they tend to cough up "air gun snot" now and again. Probably some hidden bit of the ear wax type grease they come shipped with from some hidden recess.
#2. Be sure everything is tight, esp. the fit of the barrel to the receiver and the receiver to the gas tube. If the polly port mod is just a little too high, the receiver plays see-saw on that high spot.
#3. Get some plastic ties; are going to use them as temp. barrel bands. Can bet by with duct-tape, but put a layer of Saran Wrap around the gun first to keep the sticky from peeling the stock finish. Can try one just ahead of the action to hold that in place, one where the rinky-dink barrel band is, or both. Basically, if the barrel isn't stable, it won't shoot well. YES, it will look butt-ugly during these tests., but if you learn where it needs the extra support, are on the way to a good shooting QB.
#4. Adjust the trigger. When has a better trigger ever hurt accuracy?
#5. Change scopes. Probably not the problem, but how would you know unless you swapped out for one you know is good?
#6. Try different pellets. Most rifles will shoot several pellets well, but once in awhile will get a rifle that just has odd tastes (have one rifle that shoots H-points much better than anything else, but most will shoot JSB's very well).
These take a bit of work.
#7. Remove the barrel and check the breech end for how it fits into the receiver. If those o-rings are chewed, are likely to get a inconsistent gas leak that will ruin accuracy. IF the barrel is loose-goose fit to the receiver, are likely to get poorer accuracy thn if it is tight (and he set screw tends to jam a ;loose barrel to the bottom, which opens up the passage on the top, which tends to allow a leak FORWARD at the joint of the reciever/barrel)..
#8. Push a few pellets down the barrel with a rod...you are feeling for loose spots, rough areas, or constrictions. Not a hell of a lot you can do about them, but we tend to check for them anyway.
#9. Check out past posts on recrowning and fire lapping.
#10. Check out the BREECh end of that barrel. If there is a burr or rough spot around the transfer port's passage though the barrel, it will snag pellets and cut them as they are chambered.