icloudiphotophoto-stream
I see so many pictures that I have took with iPhone on iPad Photo Stream.
But today I also wanted to activate PhotoStream on iPhoto on MacBook. So did I ! but the images it has downloaded are just around 20 photos, there are so much more if I go to PhotoStream on iPad or iPhone but on PhotoStream on MacBook there is just a few of them.
Shouldn't they all show the same thing?
Photo Stream definitively only works for photos (or screenshots) taken after it was enabled.
As for your other problem: Yes, if you are logged into the same iCloud account on both devices and have Photo Stream enabled it should sync whenever you are logged in into a Wi-Fi environment (apparently Photo Stream doesn't sync over 3G).
What else can you try:
I have found a better way. One that doesn't involve resync'ing the entire stream.
photostreamagentPhotoStreamAgent processes listedQuit Process buttonQuit buttonPhotoStreamAgent processes are no longer alive by waiting for the Activity Monitor window to show no processes with that nameWhen you restart iPhoto it will re-start the PhotoStreamAgent process for your iPhoto session and kick off a new Photo Stream sync. You'll get your lastest pictures updated and you won't have to sit through a full re-sync of all your Photo Stream.
Update for Yosemite where the process name has changed to iCloud Photos. This is the what you want to kill:
Update for Yosemite 10.10.4 where the process name has changed to Photos Agent. This what you want to kill:
You can also do this, quickly, from the command line like so:
sudo pkill cloudphotosd
sudo killall cloudphotosd
Best Answer
Apple has a fairly comprehensive troubleshooting article for isolating any inconsistencies like you have observed and clearly, the design of Photo Stream is that there is one set of pictures available from the cloud (the web interface) as well as each device that has the stream enabled.
Why not work your way through that document and then ask a second question if needed if you run into problems on a specific step? I like to power off both the devices and then start with one to work through the process to see if it's working properly, and then proceed to the next device or computer.