Hi meh, I have had the same problem when I installed my Wd mycloud 4TB for the first time.
I uploaded some picture (+270) in one folder. I could see these files from the mac were I uploaded them but bot from Myphoto, the Wd application installed on my iPad (and also on the iphone 4s).
I shut down the WD and turned on again. Then I was able to see the pictures from iPad and iPhone.
It seem it was not updating the index of mycloud internal reposity. From that moment on I have had no more this issue.
I then uploaded more than 1000 pictures and its working but it takes hours to refresh its index. Be patient.
