Related to this topic: my iPad was failing to update, every time a new ipadOS came out. The new version would download but then fail to install. So I was stuck for a long time on an old version 13.x.
It’s a 5th gen ipad, less than 3 years old, so it should still update, but it just wouldn’t.
Apple’s website suggested reasons for failure to update, like bad wifi or insufficient free memory, but none of those could explain my woes.
Then I noticed that Apple suggests, if nothing else works, connecting to a mac and letting the mac’s Finder initiate the update. So I tried it today and it worked like a charm! I now have the latest 14.4 on my ipad.