“the GTX 480 requires both a six-pin auxiliary power connector and an eight-pin one,”
Are these power connections supplied to the cards from your PSU? Each card should have 2 aux power cables connected to it, one 8-pin and one 6-pin. If you don’t have these connections, or are not supplying them with a proper 8-pin connection and a proper 6-pin connection, it may be the source of the problem.
Each card has both the 6 pin and 8 pin pci-e power connection plugged in. In the case of the 8-pin it is worth noting that I had to use a 6 pin to 8 pin adapter. That is, I originally only had 4 x 6 pin power connections, and then had to find two adapters for two of the 6 pins.
Also:
the fans on the GPUs are spinning, but perhaps the fans will still operate with insufficient power.