Thanks to an observation by Clint KA7OEI, I found and fixed a problem in the WD server which caused noise graph files to not be displayed.  

The WD server wakes up every 5 seconds, gets a list of .png files and then validates them.  But if the png was in the process of being uploaded it would be flagged as invalid and immediately deleted.  

So sites with large png files, slow upload speeds, and/or those which are configured to limit upload bandwidth might never finish the png upload in the 5 second window and thus their noise graphs would never be displayed.

I have modified the server to flush pngs files only if they are older than 120 seconds which fixed uploads from KA7OEI-1 and perhaps those of several other WD client sites.