New iPad,automatic backup not working

If you have recently updated from iPadOS15 to iPadOS16, or setup new device, automatic update will not occur until you have manually triggered an initial iCloud backup. 

This is presumed to be a safely feature that, subsequent to performing a major system version update, intentionally prevents overwriting a previously good backup. Should the worst occur, you can still restore from this good backup if needed. Only when you are certain that all is well, you can manually trigger overwriting the backup - after which automatic backup will continue.