Photos Not Showing in Develop Tab

Hello! I am having trouble viewing my photos under the Develop tab. It seems that I can view the photos in the Library tab just fine but for certain photos in the Develop tab, a gray box appears where the photo should be. I have verified that all files are on the drive but certain photos don't show up in Develop. The photos do appear in the little Navigator section and if I make an edit, like drop exposure to -5.0 (..just an example..), the edit is reflected in the little preview in the Navigator section of the Develop tab.

Things I have tried (from looking at other similar issues across the forum):

- updated Lightroom Classic 

- Preferences -> Performance -> Use Graphics Processor: [turned to] OFF then restarted Lightroom Classic

- restarted computer

- signed out and back in

None of the above solve the problem.

Something I noticed: 

- It seems that the photos that don't show up in the Develop tab are only the ones that I imported into Lightroom on my iPad then copied from a collection into my file system while in Lightroom Classic. I have verified that all photos are on my drive. I am also diligent about synchronizing my folders. 

Anyone have any suggestions or thoughts?