The auto updates only work overnight while it’s charging and connected to Wi-Fi. Are you doing that? I’m guessing it also needs to know your regular usage pattern, so it knows when a good time to update would be.
My parents each have an iPhone and iPad. Their devices are always up-to-date whenever I’m home to visit them, so it’s been working for them. They have a regular habit of charging their devices overnight when needed.
As for me, I usually update my devices right away when an update is released, but they have updated on their own when I’ve forgotten.