Stave text Properties

Hi, If I wand to make a guitar note or chord muted, I would select text mode (Ctrl T) and tupe a 'x' above the note. I would select this 'x' , right click on it to give the Stave Text menu. I would then select Stave Text Properties. There are 4 lines. I would select 'voice' 1 and then click on the adjacent channel were there is a drop down menu. Depending on the instrument, like guitar, there are a number of options with 'normal' on the top. The on I use it 'mute'. and this then mutes the notes - as in 'Palm muted guitar'.

Now for some reason the score - the guitar stave, that I am working on gives only 'normal'. It gives no other options, so I cannot 'mute' the notes. I realise that some instruments don't have this facility, but up to now guitar always has. Is there something I am doing wrong, please? Furthermore I cast seem to find anything about this in the 3.0 manual.