I've got Primax Amiga mouse, too. Mine wouldnt go up/down because of low +5 voltage (it was about 4,2V or so)
Have you tried any other mouse?
PC mouse might damage Amiga, because there is +5V line in the port. So, if that gets connected to GND... The problem is that PC serial mice have the same type of 9pin subd connector.
Does the mouse work at all? (check if buttons work, but only if you're sure its Amiga mouse, as they already told you)
The problem with mouse is almost always bad connection/cables.