timboalogo
I had the same problem - troubleshot it and discovered that it was a particular plug on the vox track that was not allowing Melodyne to start (it was crashing). Can't remember the plug, but try that.
To this point, best practice is to use Melodyne without any plugins applied. In my experience it’s been best to do the Melodyne work on the otherwise raw track, get it like you want it, then render (removing Melodyne from the equation) and sweeten to taste. Also, if you’re trying to apply the region effect to an entire performance at once (say, an entire 4 minute clip) you can run into problems. Better to split to clip into digestible chunks for Melodyne, do your work and then bounce the multiple clips to a single clip.