Are you recording 2 channels simultaneously? You can watch a third channel whilst recording two, providing it is on the same mux as one of the recording channels, or, if both recording channels are on the same mux, you can watch any other channel on any mux.
Could you please explain exactly what you are trying to do, and what you see on the TV screen.
It can depend how you change channel as I think I was caught like this when I first got the box.
If brian's post above is true (2 recordings) then in the List unavailable channels are greyed out and not selectable. However, if you use the P+- rocker outside of the List or Guide to change channels it tries to go up or down one ch at a time and it will baulk at any unavailable channel. So you have to use List or enter the channel number directly to get to where you are going.