2 ideas come to mind from my own experience. 1st one is bullet weight. Son has a 308, all the books say 168 grain is the most accurate, but his gun will only shoot 180's. 180's are sub .75" and anything else are 2" groups
2nd I have a Ruger that I could never get dialed in. I tried factory and handloads at several weights and it never gave satisfactory results. The cause was a broken crosshair on the scope