Just get the gunk out,then oil LIGHTLY they don't need much oil to run
The first 2 boxes you shot were you using HEAVY loads?
If not repeat the process with strong shells.
Firm shoulder mount of utmost importance on inertia gun. Tight to your shoulder.
What shooting position, were you sitting with shoulder to a tree or hard object? If so try different shooting positions.
Is it the pistol grip model or standard stock?
Pistol grip MIGHT need more break in or slightly stronger loads.
These guns have been extremely reliable in my experience. They're not problem guns.