Empty PhotoStream-folder on Window

Solution 1:

Basically, you need to reset the iCloud settings. There are different ways to do so.

1. Try to close the iCloud control panel and then quit all Apple and iCloud processes by using the task manager. If you reopen the iCloud control panel, chances are good your photos will start syncing.
2. Otherwise try to uncheck en re-check the PhotoStream option in your control panel (don't forget to hit the 'Apply' button).
3. A last solutions is to uninstall and re-install the iCloud control panel, which will reset the iCloud settings as well.

Solution 2:

I've found iCloud Photo syncing for Windows to be incredibly unreliable.

After multiple re-installs of the iCloud Control Panel, and eventually moving to a new PC, it never syncs photos reliably. The best it can do is show the oldest several photos that's still in my iCloud Photo Stream when I happened to install it. They never go away and the folder is never updated again...