Here is a copy of the owner's manual, coutesy of Steve's Pages-
With .22s, start with the basics- dirty, poor lube, bad ammo. Clean pistol, LIGHT lube, use some good quality ammo (CCI is about as good as you will find).
If THAT does not fix the problem get a smith to check the gun. Could be as simple as warped lips on a magazine, or mouth of chamber dinged by dry fire.
And as said, the Sterling is generally not considered the benchmark of ultra-high quality.