rebuilt did not help me as well.
Interesting thing is that I created another user account on the same iMac, and logged into Photos with the same iCloud account, and everything was synced and displayed.
However, once I go back to the original user account, regardless of how many times I logged out of iCloud and log back in, the shared photos refuse to display.
I have even deleted the photos library and recreate a new one, and still it does not work.
Coincidentally, my 500GB HDD died (its been in my 2009 iMac since day 1)
Got a new HDD, rebuilt the Fusion drive.
Reinstalled OS X, reinstate back using Time Machine, and still it is not working!
Finally, I reinstalled OS X fresh, created all the accounts again.
This time round, I pulled data from Time Machine from each user account manually to the new accounts.
And the darn Shared photos finally appeared.
I hope no one has to go through the ordeal as I did...