In most cases, this could be caused by an untriggered limit switch. Troubleshoot by the steps below.

1. Remove the glass cover on the left side of the engraver.
2. Remove the metal plate on top of the main board.
3. Check the status of LED5 on the main board.

Under normal circumstances, LED5 will illuminate when the laser head returns to its origin along the Z-axis, go out, then lights up again, and finally go out.

If LED5 is always off, the limit switch might not have been triggered. You can fix this by simply repositioning the limit switch (as shown).

If repositioning the limit switch has failed, check the wire connection beside the laser head or replace the limit switch of the Z-axis with a new, identical one.