Is that with all sounds, or just the one you are trying to control at the moment?
In the sound settings menu you can adjust the velocity curve
If that does not work, try plugging the yamaha into another midi module to check that it is doing the same to other modules
Also there is a velocity setting on the p255 (p17) http://www.sheargoldmusic.co.uk/content/Manuals/Yamaha/p255_en_om_a0.pdf