V1 Stopped working

Hi guys,

I've got a WASD keyboard that I purchased a long time ago, and it has fairly recently stopped working properly;

The control PCB's identifier is "Sydney DB V0.6(USB-inv) 2010-05-21"
The mainboard PCB's identifier is "CST 104/5/6/7/8 Main Board V1.0 2010-11-30 F2"

The problem is that several keys have stopped working:
Left ALT
Keypad 0
W
G
H
Cursor UP

I've done a continuity test on the switches all the way up to the 30pin connector, and there are no faults, the switches are still all working correctly.

Ideally, I'd like to get it working again but I'm out of ideas. Any help or suggestions?
Without the schematic for the control board I don't know what else I can test.

Thanks in advance!

Edit

I've also cleaned all of the boards and contacts carefully using 99% Isopropyl alcohol, in addition, an eye test shows all surface tracks appear to be healthy, and no components appear to be missing or damaged at first glance.