Backup to iCloud manually (Settings - iCloud - Backup - Back Up Now). Watch your iPad while it's backing up. If a problem occurs, a popup will show you why the backup doesn't complete, and you can act accordingly.
One of the reasons why your iCloud backup isn't working could be your network connection. In that case, reset network settings (Settings - General - Reset) and set them up again.
Other things to try:
Restart your iPad, or reboot it.
Remove the last backup (Settings - iCloud - Storage - Manage Storage - Backups - your iPad - Delete Backup).
Log out of iCloud and log in again.
Hope that helps.