Based on what happened to John Bowe in reply #12, it's possibly a media cache problem. Save and close your current project, then create a new one or open an existing project that doesn't use the same media as the problem project. In the new project, go to Edit>Preferences>Media and press the Clean button. Reopen your problem project and see if Normalization works again.
If that doesn't work, then go to wherever you told Pr to store your cache files and delete them. Pr will have to generate new .pek files and .cfa files, but that's the point of this exercise -- to hopefully force Pr to "reset" the audio data for the problem clips. Test again to see if Normalization works.
Jeff