How many do you have? Also, the one in your pic looks like Trilineatus or something similar, do you have more than one species of C. in the tank?
I have two three-stripe, two albino, and two peppered.
Three-stripe = Corydoras Trilineatus. "Tri", as in "Three". "Lineatus" as in "Lines".
Not sure how you feel about constructive critisism, but here goes;
Its as i suspected. Can you take any of them back? And swap for more of one species? Corys are social fish, and like to have the company of their own. He's probably not coming out as much as he doesnt feel safe. Ive noticed in my tank, the Pygmys wont come out of their cave when the lights go on until the Panda's are out first, they seem to feel a bit more secure if the big boys" go out first. Once everyone is out, they'll all split up and go their seperate ways. They only group back up if someone goes near the tank or its knocked, but they dont hide, becuase they have enough in their numbers to be happy about being out front of the tank.
This might also sound odd, but the more places you have for them to hide, the more they'll be out. If they know there is somewhere close by they can dart into, they'll venture further away. I found when i just had one cave in the 80ltr they would hang around it. Now the tank is full planted out and there are lots of places to dig under, they're all over the place. The pygmys are sometimes even swimming about with the rasbora's up top. (The pandas have also been up quite high, as iv found eggs on the top of the filter recently.)
Hope this helps