Note muting

Hi, I know some instruments don't accept muting, but many do, like guitar. You go Ctrl T, type a lower case 'x', right click on it, select Stave Text Properties and a menu appears. Select the voice you are in (usually 1). Then under 'Channel' for that voice it will say that the note is 'Open'. Use the drop down menu in that box and select 'mute', then OK. The note (and all following notes ) are now muted.
Now some scores, even for the guitar, it will not allow me to do this opn any instrument. Instead of 'Open' appearing, 'Normal' appears, and then there is nothing in the drop down menu. So I cannot mute the note. Is there some setting I inadvertently have on that prevents this?. I have selected all notes, > F8, and removed the black crosses just in case, but to no avail.

Can you help,

Henry