Excel files won't sync between Windows PC to iCloud automatically

I use a synced folder to copy local Excel files from my Windows PC to iCloud. A single large Excel file (5Mb) will not synchronise automatically when saved - the process appears to hang, and the spreadsheet eventually locks up. I can free things up by switching 'Always keep on this device' off and back on again, but if I forget I can lose the latest copy of the data.

None of my other Excel files (or other documents) behave this way: they use the same folder, and always synchronise automatically. I cannot see any reason why this one file, which has gone through many iterations, should show this behaviour when all others work - any ideas?

[Re-Titled by Moderator]

Original Title: Excel file not syncing automatically