Photo Stream is not working on my iPhone anymore 5.1

I installed iOS 5.1 on my iPhone and updated iPhoto on my Mac and everything was working as it should. However, Photo Stream on my iPhone is not working anymore. I added a photo to my Photo Stream on my Mac, and it did not appear on my iPhone. I went into Settings and turned off Photo Stream and back on again, and now the phone says there are no photos in Photo Stream, despite having almost 300 photos on Photo Stream on my Mac. How can I fix this?