Have you cleaned the barrel? Does it still group? Do the pellets come out the same speed? With HFT I have to be careful of parallax error too!
It is more than lightly operator error, I shoot a lot of prone rifle and considering the hold it is still easy to miss!!! Hanging on to long, staring down the sights to long, pressure in the shoulder, bad trigger, bad hand position, bad follow through, bad body position, even just had a bad day all makes for bad shoot!
I think clean the rifle, group test it indoors and then feeling good again go shoot your best round ;-)
I have started shooting a FT comp, thought the AG has gone wrong and all of a sudden it is right on the money again, just me! But I have also had a reg go mid comp so testing the kit has to be first.....