Have you right clicked on the PDF and selected properties to be sure it is not read only? That is all that I can think of right now for AA9. It may also be that you are looking at a file in a folder where you do not have write access, though I would not think this would appear like your are getting in this case.
I've been having trouble with this for a few days - it turned out I was stuck in PDF/A mode, which disallows most of the advanced functionality.
To switch the PDF/A mode off:
In the Preferences dialog box, under Categories, select Documents.
Choose an option for View Documents in PDF/A Mode.
Choose “never” to switch off PDF/A.
There's more info here:
Hope this helps!