Not all photo thumbnails showing.

Hi there chaps.

I have this problem with File Explorer. I have checked the box "Always show icons, never thumbnails"

But not all of the thumbnails are showing. I get a mixture of the "mountain" icon as well as some thumbnails.

I have tried to delete the thumbnail cache and nothing seems to work. Does anyone have any suggestions as to what I could do? Any help would be appreciated.

Thanks

Matt.