Take your die to the hardware store and walk into the plumbing dept.
Find a small rubber O-ring that fits down the threaded rod of the button stem.
Back home, put the O-ring between the top of the die and the stem lock nut.
Trouble is the threads of the stem itself and the die have a little play, the lock nut can "cant/tilt" the stem itself in the throat when it's tightened, pulling the case neck to one side just a bit.
Adding the O-ring allows the stem to float a bit and center too the case neck as the button is pulled through.
Viola`, an 89 cent fix,

(the O-ring trick is as olde as Granite Dies an Powder made from dirt!)
Another trick is too turn the case 120 degrees and size it again, then turn and size again. I can get cases to .001 runout with 3 strokes of the press. If that doesn't do it then theres something bad with the case.