Which firmware are you running?
I run one of my Flashforges on firmware 1.0 and one at 7.x

The one running 1.0 can read (as far as I know) any card up to 8gb - but has a problem with 'fast' SD cards..
The one running 7.x can only read the standard 2gb card that got delivered with the machine..

In conclusion:
- try different SD cards (preferably SLOW ones with only 2gb or less.. you probably don't need more anyway) - these should be cheap, so you could try some different ones of different brands - there seem to be some subtle variations between them which allow them to function on one device but not the other.
- if above doesn't work, try changing firmware to 1.0 - or maybe there is another firmware version with better SD card support.. I'm not sure about that.

Good luck and keep us updated!