RepRap discount full graphics smart controller SD card issue

I am having serious issues with getting the SD card reader to work. The reprap discount full graphics smart controller has the SD card in the screen itself. I have an MKS GenL motherboard in my Ender3. So the LCD is wired with the two ribbon cables (Exp1 and Exp2) straight into the motherboard. The connections on the back of the LCD are upside down so I have rotated the ribbon cables to match this. Everything on the screen works (buzzer, rotary encoder, reset switch, contrast pot). The SD detect pin works (which also confirms the correct orientation of the ribbon cable). It will not initiate or read the SD card. I have wired an external SD card reader using the same ribbon cable into my printer motherboard and it works fine so the SPI pins are all correctly defined and the card is formatted correctly. So software I don’t think is an issue. I thought it was just a dead card reader so replaced the screen but I have the same thing. Also I have checked the voltage regulator on the LCD and confirmed the 5v input and 3.3v output. I have also traced the pins to the level shifter on the PCB and then from the shifter to the correct pins on the SD card so now I am completely stumped? Please help.