Make sure you’re plugging in the PCI-E power connectors on the Tesla board.
Also, upgrade your motherboard BIOS, and try plugging in the Tesla card again.
If it still doesn’t work, try the Tesla card on another system. If that still doesn’t work, you have a defective card, RMA it. If it works, you might have to seek support for your BIOS manufacturer to see why the card is not posting – if it gets to that, and you know the Tesla card works, I’d RMA the motherboard instead and get a different one (assuming you can, that is, I re-read and you just recently purchased the cards)
Like ryluo said, if updating to the latest BIOS and changing slots on the motherboard doesn’t work, it may be time to return the motherboard assuming it is a recent purchase and going with another manufacturer. Do however check that the graphics cards are not the problem – test them on another system and follow the advice I already gave the original poster.
It would seem that the motherboard BIOS is at fault, since that is the underlying common factor, especially if the cards are the same… you might want to seek support from the motherboard vendor ASAP on that.