Had the same problem with my Styer.
Sorted now fitted new external adjustment system, new reg came with set. all fitted.
Then started using 7.9 express pellets, first jsb's then the new air arms express 7.9's.
I clean the barrel after every shoot just a light spray on 1 then 3 clean pull through's.
This has given me one hole groups up to 40yds and groups within 15mm - 20mm at 50yds plus.
Hope this helps.