Some albums don't have thumbnail

I pointed Plex over to my Photo Albums over the weekend and did a “Scan library files” so that imported all of them. I would say that all the photos had imported…but it’s strange. On about 5 out of the 35 folders that were imported, there’s a camera icon displayed instead of an image in one of those folders. All of the other albums have a sample image from that album as the thumbnail.

Is there a reason why this is happening? If I click on the album, there are photos that are underneath it that open up fine. I could probably manually specify an image, but was wondering why it wasn’t being properly displayed automatically.

Thanks!