LoL ok !
I dont know what is wrong with your cory but first of all it does need other corys to thrive if you have room for more but first you need to make sure it does not have a disese before adding anymore.
You said your other fish are doing well thats a good sign you will need to watch them close in case your cory has something and it spreads
Just a bunch of Questions for you:
What other fish are in there BTW?
Have you seen anything picking at the Cory?
Does the cory appear to be eating?
Can you see anything that looks like White Spots or Cotton on the fish or in its Fins or Gills?
Can you see any whitish looking slime on it. (It's light colored so it will be hard to see)
Has it been rubbing or "flashing" onto objects?
Do you have a freshwater test kit and have you tested for your: PH - Ammonia - NitrIte - NitrAte Levels?