If you're not scared of opening up the keyboard, open it up and look at the actual keyboard. As keyplayer said, look for some junk around the keyboard. And even if you don't see any, use some compressed air to clean it up and then maybe some junction cleaning spray. (Yeah, that's not what it's called. But I'm having a senior moment.)
Don't let the Casio haters get you down. When my eldest daughter started college as a music major, my father (a classical pianist and composer) went to Sam Ash with her to find a decent piano for her dorm room. He ended up picking up a Casio for her. For the money it had a good feel and good sound. It wasn't expensive, but it helped her pass piano lab. (And when she switched majors, she gave it to her sister who is now using it with her music major. Having a piano in the dorm makes theory homework much easier.)