I recently stumbled upon a problem after taking some pictures and recording some videos with my iPhone as they were not synced to my desktopn computer running iCloud services.
I couldn't tell if the data was synced to iCloud itself but at least the files didn't appear in my iCloud folder on my computer. I am running Windows 8 and 10 and this is a known problem that occurs from time to time.
Please check iCloud settings before you proceed with the solution described in this post by opening the settings:
Step 1: stop iCloud services and running tasks in Windows
Open the task manager and find the three tasks displayed here. Select them all and stop the tasks.
Step 2: delete iCloud database and settings
Open an explorer window and click into the address bar to manually type in a folder location. Type %APPDATA% and hit enter. This will open a folder that is somewhere under your user folder, usually
Navigate to the folder Apple Computer\MediaStream and remove everything that's inside this folder.
Step 3: log off from Windows and re-login again
This last step is necessary to restart the iCloud services again. If you are not sure you can of course reboot your computer.
After that you will notice the iCloud photo folder instantly getting in sync again as the missing media files will be added immediately.