Choose your pellets very carefully.
I bought Barry's FTP900 and found it to be extremely pellet fussy. I don't know why this should be as Air Arms have been using Lothar Walther barrels for years and although you need to find the pellet a particular rifle 'likes' it has usually been pretty straightforward, but not with the FTP Barry and I had.
I would suggest if your groups have suddenly fallen to pieces, the first thing you need to do is clean the barrel, then take it to the chrono.
"I'm your huckleberry, that's just my game"