Corys are naturally nocturnal feeders, although my pandas will feed whenever food is present. They are pretty good at finding food. If you are worried about them not getting enough food try feeding them just before you turn off the lights.
Also, lets rule out any other causes of stress.
What are your water parameters? pH, ammonia, nitrite, nitrate?
Are there any signs of distress or illness? Are their barbels looking good?
Can you give some other details about your tank?Filter? Whats your substrate? Plants? In your other posts you have mentioned fish other than those mentioned in this post. What is your stocking?