This is interesting because I have the opposite issue with my newly purchased ATV (3).
I have over 200 photos in the Photostream of both my iPhone and my iPad but only 6 are appearing within the Photostream of my ATV.
Reading around this a little, it seems that Photostream will only hold a photo for 30 days, after that it is deleted from Photostream on iCloud. However, if your idevice downloads it in this time it stays on there. But the ATV seems to only read the current Photostream, it doesn't seem to store the photos locally. So will only display the last 30 days of photos.
Can anyone confirm if this is correct please?
If so, not sure at all how you have got to your predicament!