Gunpower SSS extremely inaccurate

Hello,

Im new here, hi to all.

Im new owner of Gunpower SSS 5,5. Great gun, but my is extremely inaccurate. 10-15cm groups at 30 meters.
I try full range of JSB pellets – 0,87 RS to 1,6 grams monsters and all power settings.
At low power are groups little better (8-10cm)

Have someone this problem too? Where will be problem? What can I try?

thanks to help me

Luda