I just bought a Yamaha YPG 625. I've never played a piano before but the last five or so keys on the far right side of the keyboard do not "sound". I've adjusted the volume up. I can hear the keys thump but can't get them to produce actual sound (or it's really light). Is this supposed to happen?

All of the other voices sound fine..Bright Piano, Organ, Flute, etc. etc. I even get good sound on the Grand Piano except when it comes to those keys. Is it because they are so high in pitch?