flex time unavailiable for project

I am trying to use flex time on a vocal track in my project but it is completely unavailable. When I click 'show/hide flex' in the tool bar nothing happens, nothing at all. no options to choose flex mode, no analysing, nothing! This is a complete mystery to me, I can open a different project and flex time works perfectly. What am I doing wrong that flex time is completely unavailable on this project?