Have you done the free Windows 8.0 to Windows 8.1 upgrade, then noticed your iCloud Control Panel has stopped updating your Photo Stream? Even on the latest Apple iCloud Control Panel 3.1, the issue doesn't just go away. Somebody named "Alex leong" noticed this problem, and asked the Apple Support Communities for help, back in his Sep 23 2013 post. Over 20,000 views later, and turns out Alex himself wound up fixing his own problem, kindly posting the procedure he developed on Nov 05 2013. You can see for yourself by scrolling all the way down in the same forum thread here, but I've also quoted his steps right here:
1) Kill these processes:
ApplePhotoStreams.exe, ApplePhotoStreamDownloader.exe, APSDaemon.exe, iCloudServices.exe, iCloud.exe
2) Delete this folder:
3) Launch iCloud Control Panel. You will notice that Photos is not sync. Enable it and apply it.
4) Now launch iCloud again, and turn on the "Photos" checkbox. That's it, the re-download of the thousand last photos you've taken resumes, right away.
I've also created a video of me doing the above steps that should be helpful for you if you're struggling to implement his fix, seen below.
There are a lot of reasons that iCloud could break. This is only one fix, done at your own risk. But it does seem to work for a lot of people, including 2 laptops in my own family. If it goes well for you, please drop a comment below to let us know. I cannot reply to all follow-up posts of the sort that say "it didn't work for me" since I likely won't know why, since it _did _work for me. So I'm politely suggesting you may want to instead document your iCloud issues over on the Apple Support Forums instead. Thank you!