Please help

I have a lovely Commodore mouse (for the PC) that is serial interface and it just works on the vertical axis
I have tested the wires with a multimeter and all are connected (and if the data wire was broken probably it would not work at all)
The buttons seem to be working fine too

when I poke the mouse PCB sometimes it works correctly on both axis for a few moments
I have been looking closely and I dont see any crack on the PCB
But unfortunately I cannot remove the PCB as the axis rollers (I dont know the name) are plastic soldered to the mouse case

what can it be ?
Does anyone has even had this issue ?

Thank you

Nuno