Thanks to google and I found another post with the info, so this is the content of post and not the link.
IMPORTANT: for very large uploads (such as a complete and full iOS Camera Roll,) the iPhone/iPad Auto-Lock control needs to be set at a high value (meaning, set to Never,) BEFORE the upload begins, or else Auto-Lock will likely time out, step in, and shut down the
iPhone/iPad, and your uploading will stop before it completes.
(WD Support says Auto-Lock is a battery-saving design feature of iOS devices.)
Also, for large uploads, have a fully charged iOS device, and keep My Cloud app in the foreground during uploading.
Be sure when all is completely finished to put your Auto-Lock control back to where you had it set (at 5 minutes or under).
Subsequent auto-uploads may not require resetting Auto-Lock if only a few items are uploaded.
I have been told by WD Support that Android devices do not have this issue.