An output functions when locked to one pin, but not when locked to another.
If the equation in question functions properly on a different pin, this indicates one of three things:
1. Something else on the board is keeping the output from toggling. Check the connections and traces for shorts. Also, verify that the downstream device is not holding the trace at a particular level.
2. The original output pins may be damaged. Create a test case to verify that the output pin is still functional.
3. There may be an incorrect connection in the device. This could be a software or a hardware problem. Verify that the equation in the fitter report is correct.
If you suspect your part is damaged, please contact your local Field Applications Engineer or Xilinx distributor for more assistance.