Tuning Solving Issue with Silver Shadow Supreme Rifle (Gamo)

Random lame guess is a bad piston seal (could also be a bad spring??) in which case is just needs a new one, cleaned and lubed right and broken in again. A company called ARH likely has a piston seal that will work, probably a new spring too should it need one. They also have very good moly paste for relubing before reassembling.

https://www.airrifleheadquarters.com/page/page/251485.htm

Here's a diagram of the rifle which if you take your time, should be semi easy to tear down. You should also find plenty of help here in how to tweak and tune a spring piston air rifle. 

https://www.airgunspares.com/gunspares/airgun-spares/gamo/air-rifles/s/silver-shadow-supreme.html

Depending on where you live in the country, you could also try contacting some of the pro tuners out there to see if one of them would work on it or not.