Cories don't really seem to relish algae wafers. My green ones prefer Hikari sinking carnivor wafers,the Tetramin bottom feeding pellet, and Wardly shrimp pellet. They might not seem to be eating because, like CatfishMike says, they're glutting themselves at night. Also, if they seem content to nibble on the stuff already growing in your tank, then that could be why they ignore the algea wafers you place in the tank. Also, try smaller bits of food, and place that food in a weighted cup. Sometimes cories have a hard time digging stuff that has fallen between grains of gravel. I had a tupperwear lid in my tank that I placed bottom foods on for the longest time so that the cories didn't have to root it out. It also makes for a cleaner tank.
~~Colesea