WhatsApp doesn't back up

I have auto back up on but there wasn't a back up since 16th December. I spoke to Apple who advised me to delete the data and try it again but now it says there's never been a back up and it still doesn't work.

I've tried everything I can think of. Reset network settings, signed out of iCloud and back in, restarting my phone, deleting data. I have more than enough data in my iCloud so that's not the issue. I've spoken to WhatsApp who have been no help and I'm just out of ideas.