Hi Yorgi,
It sounds like you may have two staves on the same channel. You can check in Staff/Setup, and see if a particular channel is assigned to more than one staff. If it is, and you don't have "special" assignments due to the MIDI playback device you're using, just click the "Chnl" button at the top of the column and then click "Ok." This will reset the channel assignments to one per staff to avoid such problems.
If that doesn't work, please post a copy of the file, or send it to me at
support@notation.com and I'll take a look at it.
ttfn,
Sherry