Is the Greener GP one of the real Greener trap guns or is it a standard GP that you use for trap.?
What load are you using in it? .... ?fps and weight.
And have you had your shooting style with the gun checked by an instructor?
One of the old Greener GP trap guns from the 1950's, 1 oz of 7.5 @1150 - 1200. No to the instructor, but several experienced shooters have watched and they aren't seeing anything obvious as to why I am missing*. The few birds I do hit are mostly straight away with minimal climb. We are shooting wobble trap from the skeet pads.
* Give the patterning and the DAC & DAH I think it is mostly the fit. I may try again today, the last time I shot the LOP was 15" with the slip on pad, which is really too long for me. I took two of the foam rubber spacers out to get back to 14" LOP.