iCloud Photo Sharing not showing up on my mac anymore.

I've been adding photos to a couple iCloud photo sharing albums for the past month mainly from my iPhone but also from my macbook. Everything was fine and they were in sync in real-time. Now, I added some photos from my iPhone again but they are not there on my mac. I also added photos from my mac to iCloud photo sharing and they do not show up on my iPhone. What's going on? How come it worked yesterday but not today? I can still see the activity and comments, but the photos are grey.

Please help! Thanks.