My relatively new iPad, having worked very well for its first 5 months, has stopped syncing to iCloud. iCloud Backup is set to on, and I have plenty of storage space on my iCloud. I have left it plugged in, connected to WiFi and locked many times in the last couple of weeks, but no backup. When I tell it to Back up Now, it shows me the first milimitre or so of the horizontal blue activity bar, and the message "Estimating time remaining..." but it gets stuck there. Last night I left it doing that overnight, and when I checked this morning the process had advanced as far as half-way along the horizontal blue bar. Within a couple of minutes the blue bar had gone back to the beginning, and then it stopped altogether. The date of the last backup showed again as two weeks ago.
I cannot find any relevant advice in the User Guide.