How can I get all photos in PhotoStream on my iPhone?
I don't know if this is an answer, but I wanted to add some images, so:

First, what computer / os do you use? Can you see the photos on this computer? If you are using iPhoto or Aperture and the photos you are looking for are not visible in these programs, they have already been removed from the iCloud server and cannot be re-synced to iPhone (at least not through Photo Stream). iCloud only stores the last 30 days worth, or 1000 photos, whichever limit is hit first.

On the iPhone, go to Settings then iCloud (as seen in the first image below).
Next, tap on Photo Stream, and ensure the "My Photo Stream" slider is set to ON (as seen in the second image below).




Finally, (as this is iCloud (grrrh), it might help to turn these settings off, wait a few hours, and turn them on again. I had to do this recently, with photos and mail. Hope this helps.