Photos on Windows 10 do not upload to iCloud
I have Photos on a Windows 10 machine and am using iCloud for PC to sync with my Mac devices (iMacs, iPhone). The problem I have is that Photos does not seem to upload all images to iCloud (it does a small and apparently random selection for some reason), and neither does it reliably download. Huge gaps appear in the library, usually of a period of a month or two.
I have plenty of storage space available in iCloud and syncing between Apple devices is fine.
Any ideas please?
iMac, OS X Yosemite (10.10.3)