Same problem; however, I could add new (and I mean fresh off the camera, ipad, etc) to the stream... which indicated to me that, as someone said early on in the chain, photostream 'tends' to drop old photos when forced to update. But it seems you have to force it to update. Maybe...
To try this out, I created a new album in iPhoto. Selected all photos (old, and new) in the iCloud My Photostream shared album. Dragged them to the new Album. Then deleted them from the iCloud My Photostream album.
In a couple of minutes, my Apple TV showed that I had 0 (zero) items in Photostream (no logging out of anything).
Next, Reverse the process and copy them all back in.
Apple TV now plays all of them.
In a month I imagine I'll have to do it over again, but I can live with that until Apple has the wisdom to make this work as we want, rather than as they intended.