Hmmm well i have never had this problem.
Couple of things to try is iPhoto>Preferences>Photo Stream and make sure "Send all new photo's to My Photo Stream" is checked.
You can try turning Photo Stream off @ >System Preferences>iCloud restart your Mac then navigate back there and turn Photo Stream back on.
You can repair permissions via Disk Utility via Applications>Utility Folder then Click on Macintosh HD in the left column then the "First Aid" Tab in the window and then Repair Disk Permissions down the bottom of the window.
Let it do what it has to do (don't freak out at all the errors in the Log Window) once its finished restart the Mac.
Then if all else fails, try make sure iPhoto is shut down completely then goto ~/Library/Preferences and look for com.apple.iLifePhotoStream.importstatus.plist and drag it to the Trash (
DO NOT EMTY THE TRASH AT THIS STAGE. Log out and back in (No need to restart)
Now reopen iPhoto and see if your Photo Stream is all good. If there is no change, goto trash Option+Click on the .plist and in the contextual menu select put back. If you get a "There is a item by this name there already" Select Replace.
If that hasn't worked goto ~/Library/Preferences again and look for the next .plist witch should be beside the other one com.apple.iLifePhotoStream.plist and do the same as you did before.
Now if it is still not right, come back and see us again
HTH
Cheers
EDIT : the easiest way to get to that Folder then file is open Finder the use this Keyboard Shortcut Shift+Command+G and it will bring up a dropdown at the top of the window, then just copy and past ~/Library/Preferences into it and then click go. Apple hides the ~/Library/ Folder by default because if you mess around in that, they believe you can do bad things to your System, which you can if you don't know what your doing
