If the CP300 is the same vintage as my CP200 (about 10yrs old?) then I think the keybeds
for that range of Yammys are known to have this problem. Google "yamaha sticky keys" and
see what you get.
Yamaha seem to acknowledge this and some folk have had
success in having the entire keybed replaced. I just take it apart, swap the keys around
and/or give them a good wiggle
the CP200 is my gigging 'board that gets pretty poorly treated anyway!