YAMAHA MO6 attack sound

Hello everybody:
My MO6 has a trouble:
Everytime I leave the keyboard a few seconds of no playing, when I play again, the first note played, sounds with a slow attack, that is very notable, in voices like piano, guitar etc.
I already did a factory reset, but the problem persists.
Would anybody please help me to find out if this is an electronic damage or is there any solution via programming.
Thanks a lot.