Wave Forms in Timeline Disappeared

Some of the clips in the timeline no longer have the wave forms visible. I've tried two suggstions: 1) the option to deselect then reselect the Show Audio Wave Forms option and 2) the option to render the audio. This works....sort of. A separate audio file is created, which seems a bit clunky. Isn't there any easier way to have the wave forms re-appear? If option #2 is my only choice, what will happen if I delete the new audio file that is created?  

Thanks!!

Scott 

P. S. What makes it even weirder is that I can have two clips from the same original footage and one has the wave forms and the other does not.