In slave mode, sensor will only follow az/el commanded (where to look), until pilot manual command input, it's not point stablized.
After you move TDC in slave, since it's not point stablized, you need press S2_Press to ground stable the sensor, then move TDC over target to auto lock target,
or S2_Press when target is centered in the image(if luckly enough).
Basic rule behind the logic: target designation has to be up to the pilot.
Avionics make it easier, but should not be automatic.