I think you need to get a picture up.
I would look for some sort of damage to the disc or mouth, any irritation.
I would also buy some new test kits and forget what you know, work through ph, ammonia, nitrite and nitrate just to rule all of these out.
Any new substrate or ornaments in the tank that could be causing irritation?
Have you ruled out it having anything stuck in it's mouth, I tried an ancistrus in my tank which I was advised against and my rays ate it....I feel silly as this could have stuck in their mouth and the pectoral bones can cause injuries.
I assume this was not wild caught on a hook?
Hope these rambling questions help, sometimes we need to rule out the obvious.