Just wondering - did you try deleting the plugin cache, that usually sorts problems like this out for me ?
However - actor thumbnails become a little bit of a mess with plex if you use multiple sources across your libraries. For example, if you use TheTVDB for your TV Shows and TheMovieDatabase for your Movies then the thumbnails start to conflict with each other and some end up displaying blank. I created a thread on this a while ago but never got a conclusion and I just look at it as a limitation -> Weirdness with TV Casts Thumbnails
What I ended up doing was switching the agent for my TV Shows library to use TheMovieDatabase so I would not get this blank thumbnail problem.
