There are several things you should look at or try, my first thought is bedding, some rifles are extremely finicky, Meaning everything has to be perfect, I would suggest next time you sit at the bench fire a group as you normally do, then completely remove the rear guard screw, then fire another group, if point of impact changes more than a inch or two you have less than perfect bedding, if you have a perfectly bedded rifle point of impact shouldn't change, I have several rifles I can completely remove barrel and action that poa will not change when reassembled, primers can and will make a big differnce in some rifles, so a simple primer change can correct that at times, my 2506 liked reloader 22 and a cci 200 primer, the testing of magnum primers regardless of manufacturer produced larger groups, when reloader 25 hit the market this powder not only produced higher velocity (100fps) but tighter groups, only this time my best groups came when using the Winchester magnum primer, one lat thing I would try is a different make of brass, I've had two rifles that went from wonderfully accurate to oh crap somethings wrong, another I would try just to see what happens is contacting the lands and see what happens, I've got a ruger number 1 in 257 Ackley that shoots best making 5 to 10 thousandth contact, I'm sure there is a cure for this problem but these days of component shortages and high prices finding a cure could become costly, good luck.