iWork files not syncing in El Capitan

Hello,

There seems to be a problem with my iCloud drive.

Whenever I create a new file in either Pages, Numbers or Keynote and save it to the iCloud drive, nothing seems to happen.

There appears an 'uploading' sign under the files in the drive but nothing really happens, they do not sync with my other devices.

Apart from iWork files, everything syncs perfectly (like PDF's, Pictures etc.).

What could have gone wrong with the iWorks files?

I tried uploading the files directly from icloud.com and they upload successfully and sync with all my devices.

This happens with my MacBook's iCloud Drive only, which is 15" late 2013 top-end model running OS X El Capitan (10.11.4).

Thanks,