Umerex p99 co2 downgrade issues

[Nicky blue eyes]

I got this pistol and it was firing at 380 fps.  Looked on good old youtube and found 2 different methods of lowering the fps. How ever neither was in english. The method I used was to put  an o ring in the gas chamber to reduce the amount of co2 going in.  The slide only slightly goes back now. Not really a problem because it dosen't rely on the slide to operate the system how ever it now seems to have lost some distance. Is this normal? I thought it might be leaking but I couldn't hear any hissing. This is my first gbb pistol so I'm not sure what i should be looking for.