Some of my loops playback just fine but their wave graphic is missing.  They appear to be silent tracks.  When I first put them together, the waves showed up.  I saved, closed X1 Producer, then came back later and the waves had vanished; leaving me with flatlines.  They even appear flat in Loop Construction.

Please help.

Select all clips (Edit > Select All).  Then right click one to choose "Associated Audio Files" (or press SHIFT+K).  Click the button labeled "Recompute Picture(s)."