First thoughts:

You haven't got a gas ram fitted? - They lose power like no one's business and (apart from the original Theoben and HW90) being sealed, you can't increase them back up.

Check the breech seal - If everything else is okay (no graunching or difficulty cocking), sometimes a simple breech seal replacement is all that's needed.

I'm not too familiar with the SuperSport but - it might need a new seal on the piston (are these O rings?)

Have you changed pellets? - believe it or not the variations you mention can be down to a change in pellet type.