One of the major causes of issues with the Zega Mame Boy Advance SP kit is improper soldering. Consult the GPIO pinout and try reflowing all pins associated with your particular issue including:
Over 99% of issues so far have been caused by soldering issues when attaching the Raspberry Pi to the mainboard.
Check the ribbon cable is orientated correctly and try reseating. If in doubt please email me.
This means that while the power supply to the LCD is good, the video signals are not. Check all video signal solder points according the the GPIO pinout