The slots depend on your make on mainboard,however I assume it would go in slot that it fits in and closest to main power supply.I would of assumed you had fitted it in the right slot?This could be a bad connection,ie card not fitted correctly?Make sure it is in right slot,I also asuume white slots are smaller than the black slots?Make sure all the cables are connected to the right connections on MB.There can be ports on a mainboard that look same size and are used for other things not your Graphics card.
As I said double check the way you fitted this card it could be a simple wrong connection,card not pushed in correctly,cable in wrong port,etc?
The white slots on my MB usaly are for network cards and extra USB pci.





























