sometimes the autofocus calibration fails when it had been calibrated on different data before, because:

1. previous calibration cropped the autofocus sensor area to the relevant area where the signal is visible
2. a different plate may introduce refractions that move the signal on the sensor in the y direction, hence moving out of the previously cropped region
3. if this signal moves far enough, which in practica can occur, the calibration may fail, seemingly randomly.

diagnose how:
when calibration fails, manually investigate the autofocus signal. this may show 1) no signal in the cropped region, or 2) clearly shifted signal in the shifted region. [3) if there is no crop, this issue does not apply].

proposed fix:
remove sensor crop, then calibrate.