Is this installation already a few years old?
And if so, did you perhaps manually select posters of the affected items?
And did you convert the library to use the new “Plex Movie” metadata agent?
If the answer is Yes to all of the above, you might want to unlock these old manual poster selections. They were probably set ot a poster which was available through the old agent, but is no longer there through the new agent
Trigger a Refresh Metadata of the whole libaray after the procedure.
