If you have an Android device and have set up the My Cloud app to back up your photos, you are very likely losing many of your photos.
If you delete the most recent photo on your Android device, the next photo you take will not get backed up. This is because the new photo gets the same name as the deleted photo and My Cloud mistakenly thinks the new photo has already been backed it up.
I have contacted WD support and they have confirmed that this is the current behaviour of My Cloud. They gave no indication that they were taking steps to fix it.