Is the ATC in mid cycle ?
make sure that it is at seq#1 or seq#3 ( tool pre-select - it may be even #5, not exactly sure ).
Is the rapid over-ride turned down to zero ?
Do you get any distance-to-go values when executing pallet change ?
Z &/or Y ( or both ) must be at the reference point before APC cycle can proceed.
Is there any messages on the "Diagnosis" page regarding conditions or signals not yet met?
Does it time-out ?