Hi Paolo,
If David's suggestion doesn't fix the issue, it may be that they keyboard you're using is only transmitting on one channel (probably channel 1, which is typically the piano). Please check your keyboard manual and see if there is a setting to allow multi-channel transmission. I had an old keyboard at one point where I had to set the transmission channel for every time I'd play into a staff. I no longer have that unit
ttfn,
Sherry