Some of the many causes would be keeping them at a high enough temperature to make discus happy, degraded water quality, having a single cory which would seldom be very active, significant changes in diet that might affect him, wrong species for the water conditions, any kind of illness, etc. etc. etc.
How about some basics like what cory do you have, how many of that specific species do you have, what are the tank water parameters, how many fish are you keeping and how big is the tank, what temperature is the tank, what do you feed and how often, any recent additions to the tank such as new decor, new fish and so on.
With this kind of information we might have a clue where to start helping you.