Document thumbnails can sometimes be missing in the Library page when the document store is on a mapped drive. To prevent this happening document thumbnails can be copied to a folder in the local directory which is cleared when Skyline if restarted.
To copy thumbnails to a local directory:
Log in with Host rights and go to Admin.
In the Portals section click Default Configuration.
Find Library - Cache document thumbnails in the list.
click to set the option as True so that the document thumbnails are copied to the local wwwroot\ThumbnailCache directory.