I was exporting Pages files to PDFs and not changing the name....everything before the extension stayed the same. I renamed the pdf (added pdf to the name and it immediately synced. In some cases, I can open a file in a folder that is not syncing and it will upload. Watch the progress indicator in the sidebar.

I don't recall the exact wording but it was something like....It is working for me and I am using the 14.5 iOS beta. I didn't really want to wait....this problem has been ongoing since September 9th, 2020 and the users on the thread are rather irritated with the situation. Here is the thread FWIW : https://discussions.apple.com/thread/251815788