I suspect the Z-axis isn’t sufficiently taut so as to hold against the springs.
Please check the Z-axis tension and verify that the belt is in good condition. If you need a new belt, you can get one by purchasing the Maintenance Kit:
(be sure to let shipping know that you have an SO3 w/ a belt-drive)
Unfortunately, upgrading the Z-axis to a Z-Plus or HDZ pretty much requires:
- replacing the controller to a current version which has the Expansion Adapter necessary for the proximity switches — this will require a new electronics enclosure
- adding Molex KK connectors and pins to the motor wiring so that the motors can be plugged in to the new board — see: Shapeoko 3 Connector Part Numbers