1. Check the PCIE ports for dust/messed up pins.
2. Check the GPU's for deformed PCIE inserts.
3. Take the bridge off and try to run it (IF it detects the second, the bridge was messed up)
4. Swap the GPU placement, launch and reinstall drivers clean and new
5. Make sure that the second GPU was detected by windows and the driver WAS infact installed (If windows doesn't detect the second GPU, it can't properly activate SLI)
6. (Last resort) Wipe the hard drive, and perform steps 3 and 4.
Hope it helps.