Its most likely locked poster fields as you have manually selected a poster at some point in the past. The Reset all manual poster selections in a library at once or unlock other metadata fields doc should help with that.
If they are considered locked then unfortunately yes you’ll most likely need to unlock them 
If its just movies and you have 1 library then its just 1 command to unlock them …
curl -X PUT "xxxx:32400/library/sections/yyyy/all?type=1&thumb.locked=0&X-Plex-Token=zzzz"
replace xxxx with your ip, yyyy with your library id and zzzz your plex token. If you want to be safe maybe take a backup before running 
