I've had this happen a few times now, not sure what causes it, but iCloud Photo Stream occasionally stops syncing on Windows 7 for me.
The way I have fixed it is to reset iCloud by:
- stopping the Photo Stream process ApplePhotoStreams.exe using Windows Task Manager
- renaming the
%APPDATA%\Apple Computer\MediaStreamdirectory (i.e. C:\Users\username\AppData\Roaming\Apple Computer\MediaStream)
- log out and back in again
- restart iCloud (Start, type iCloud, press Enter) and re-enable Photo Stream.
Judging by this thread on the Apple Forums uninstalling and re-installing iCloud also seems to do the trick, but of you're reasonably computer savvy, my method may be a little bit quicker.
With any luck, the next version of iCloud will fix this issue.