I spent a great deal of time troubleshooting this issue and then, quite by accident, tripped upon the answer. At least the one that worked for me.
Apparently photo stream will not sync if the battery level on your device is below a certain percentage. Don't ask me what that percentage is yet as I have not done extensive testing in that department. That being said if you are having problems syncing a device simply plug that device in so that is is charging and make sure it is on a wifi connection. If you are syncing from an iPhone to a iPad I found it goes faster if they are connected to the same wifi network. However, this may be my imagination getting the best of me. My guess is that Apple does not want the device to fade off during the sync process causing a corruption of data or the library file it is editing.
It would be great if we could get an Apple engineer to confirm this (hint, hint all you Apple engineers out there). It would be even better if there was some sort of popup message that alluded to this tiny yet incredibly important detail or even better a button that initiates the sync and then gives you the popup message if you are not connected or low on battery. Just suggestions... not saying we don't like your technology but if multiple users are having the same issues it may not be as clear as you think it is.