Cannot see photos, most are exclamation marks. Why?

I just started using icloud to store my photos. I turned on icloud photos on one of my old phones and waited for all the photos to be downloaded. After seeing my phone say the download was complete I tried to see the photos from icloud.com, but some photos are missing and most that have downloaded only show up as exclamation marks when I try to access them via my computer on icloud photos. I see no way to edit the photo or reason for it not showing up.