Only photos in taken on your phone after turning on Photo Stream will be uploaded. Any photos taken prior to this will not be uploaded. If you need to transfer these photos to your other devices you can either import them to your computer an upload them to Photo Stream from there, or you can use an app like PhotoSync, which will also wirelessly send them to your other devices.
I turned on Photo Stream and took 2 pictures. One uploaded and the other one did not???
They only upload when you are connected to wi-fi, the camera app is closed, and your phone has at least 20% battery charge. Perhaps one or more of these conditions wasn't met yet for your other photo.