Is the
dimmer possibly set to be on constantly, aka hot patched? Also, try checking the console making sure the
channel in question isn't parked on or the dimmer number isn't profiled to be on at full all the time. You can also try taking out the control card in the dimmer rack and making sure everything looks fine, clean the contacts a little, and then put it back in making sure its seated correctly. The last thing like you said, the dimmer may be bad. I had a dimmer once that would only dim between 0-20%. At 20% it was on full, so it made programing interesting to say the least. People would look at the console and wonder why that channel was at 15% and the others at 75%.
I know that NSI rack mount dimmers have that problem fairly often, some times the light is on full blast all the time, some times it won't come on at all but will in another dimmer. The fix is just pulling out the control card, cleaning it, and making sure its seated properly. Same thing with the old ENR Colortran racks.