How can I force sync iCloud Drive?

[iMore Question]

I have iCloud Drive installed on 2 Macs and a new MacBook. All my documents are saved on iCloud Drive.
I can see a pie chart symbol appearing next to iCloud in the Finder. Whereas it used to zip through the syncing process, it now sits there frozen. It says: '3 more files to upload, (0KB of 1.8MB)'.

I Recently installed El Capitan on all machines. My iPhone has the latest IOS version.