Faulty polyphony on consecutive notes

by Fabio
(London UK)

Hi, I have a Yamaha P35 and noticed yesterday, on all octaves, between adjacent F&F# and B&C if I keep pressed one the other is irresponsive.
Each octave the same, if I play the 2 keys within the same octave.
It seems not far from the issue discussed here somewhere else, although not played together all of these notes work fine.
Scratching my head really.
Thanks for any help