Sheet set plotting with page set-up override

Most of the time when I use Sheet Sets I have the option "publish using page set-up override" so I can select one of a number of plot options.
Occasionally though I find a particular sheet set has the "publish using page set-up override" greyed out.
Why is this available on most of my sheet sets but not on all?
I am using ACAD 2007.
thanks
Benny
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