Transferred Library To A New PC Now Not All Poster Art Is Showing

I just transferred my Plex library to a new PC. I keep all of my media on an external drive. When I installed Plex on the new PC it went through the update process. About a 1/3 of all my films just have screenshots now and no poster artwork. However, when I go to edit the title the Poster Art is there I just manually have to add it. Any help would be sincerely appreciated.