Depending on your setup even piers can go off over time due to wet weather, dry weather affecting soil and it can create stresses in the soil that could shift a slab slighty or a pier.
I have not particularly noticed that with my setup but my pad is 1 metre deep x 800 square of concrete with lots of steel and a large flange of concrete on the top level.
But if my guiding started to go off the first things I would check apart from the usual guiding settings would be balance, cables, polar alignment - the physical side of the setup.
A quick 30 point t-point model would show if PA is off. I think you can save the existing model if it is large and reinstate it if PA turned out to be fine.
Greg.
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