I battle this with my .177 TX constantly as well, it will be sighted in nicely and in its case it goes. Pull it out a week or 3 later annndd it's a bit right or something. Do I adjust windage,...hold different, is it wind, barrel, JSB pellets, too much coffee, not enough, the price of gasoline lately.....whhhhyyyy?? Then I calm down and leave it as is. I bought it and shot it through winter (mostly just above zero degrees here) Groups beautifully @ 30Y but it's always off a tad, then mysteriously the group starts heading towards the bulls ever so slowly.

You switch back and forth from irons to scope alot it seems on your guns eh?