Q: My photos are not uploading to a shared album in new Photos

I too was having this problem, though it has corrected itself over the past few hours.  I have a number of shared albums created in iPhoto on an iMac.  I installed 10.10.3 on an MBP.  From there I started the Photos for Mac app.  It imported content from iPhoto.  iPhoto was populated with the shared albums.  I installed Photos on Friday, and then did not look at again until Sunday.  Initially, only a few (say, 5 of a total of 15) shared albums were populated with pictures in Photos.  The shared album names were listed, but the album itself was blank.

By chance, I clicked the Get Info command under View in the menu.  I noticed, via Activity Monitor,  this action caused the "Photos Agent" process to fire off. The Shared Albums started to populate.  I was hoping I had solved the problem, but when I returned a couple of hours later only an additional 5 or so shared albums had populated, not the entire set.  Doing a Cmd-I on a blank album would no longer start Photos Agent as before.  Consequently, I went back to the original iMac, and opened up a few of the original shared albums and just generally clicked on a few random pictures.  My hope was that this activity would somehow trigger some flag in iCloud that the shared albums were potentially different.  I then went back to Photos on the MBP, quit it, and then started it up again.  The Photos Agent app only seems to be initiated when Photos starts.  I did this a couple of times (allowing time for Photo Agent to pick up new batch of pictures from iCloud and populate the shared albums).   At this point, my Shared Albums are completely loaded in Photos.

The trick, if there is one, seems to be to get Photos Agent to fire off to look for pictures.  Though Cmd-I worked to some degree once, I found the more reliable solution to start and stop Photos multiple times.

Hi

I have this problem too, I'm trying to share 100 pictures to an album it shares 3, 4 or 9 sometimes and stops! so frustrating!