I have a Windows Laptop with iCloud Drive on it too, also a Mac and various iPhones and iPads. All of them work together, but I notice that the Laptop version can be very slow to sync. Most of it is synced but some folders take longer than others.
In fact there are 4 in the top of the list when I sort the column that say Sync Pending.
So on investigation I see in those directories that some items are synced and some are not.
I have a LOT of data on my drives, including iCloud Photos which I'm not worried about. It works fine.
I did a test file and placed it in one of the four folders that appear to be stuck, and sure enough, it's the one that's stuck.
It's on my laptop, but no where else. It's not on the icloud.com -> icloud drive, nor is it on my other devices nor Mac.
I am logged in to iCloud on the Laptop.
I can open the file on the Laptop in it's appropriate app
But I can't see it anywhere else.
So I'm suspecting that because of the LARGE amount of that I'm syncing on this new setup, it's just taking it's time.
I have only just enabled iCloud on the Laptop in the last week.
I have a very fast permanent connection so that's not the problem. I think the iCloud drive app is just throttling the transfer of data somehow to not flood the link?
It's annoying, but not unexpected. iCloud is brilliant between Apple devices, but seems to less than optimal when talking to a Windows box. Although having said that, it is a lot of data I'm trying to sync.
Watch this space as they say.
... and no, I"m not falling for the old sysadmin trick of telling someone to reinstall their entire system again.