Not able to sync all photos to PC from iCloud

I've setup iCloud for Windows & my iPhone 7 as per the instructions in various forum threads to enable photo syncing from iCloud -> PC.

Within Windows Explorer on my PC, I select 'iCloud Photos' then the option 'Download Photos & Videos'. I select then to download 'All', in my case this is all photos/videos since 2013 from my phone. In the 'Downloads' folder within the iCloud directory I can see that 5 new folders have been created as I would expect - 2013, 2014, 2015, 2016 & 2017. However, even after several weeks since this was first setup, only 700 photos have been uploaded to the 2017 folder and none to any of the previous years. I have over 4000 photos/videos on my phone. This upload of 700 photos occurred within the first day of initiating the option 'Download Photos & Videos'. Phone/PC running latest software.

Any help would be much appreciated.