I had that problem with a Weaver 2.5 X on my 375 H&H Model 70. I put the scope on in '74, zeroed it and no problem.
Then about 5 years ago it wouldn't group. Tried to re-zero...........nothing, no luck.
I fixed it by tossing the 2.5 in the trash and putting on a Weaver K4.
I wont have a scope I have to play games with when I zero. If I click up or down, I want my impact up or down. I don't what to go south X clicks and then north X minus 2 or what ever.
That isn't the way scopes are suppose to work. I certainly don't want to go afield with an "iffy" scope. If I don't have confidence in my equipment, I wont shoot well whether the equipment is on or not.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071