I am new with this plugin (I've just installed today) and I had the same problem. Firsts movies added ok. Then added a new set of movies and didn't appear in "media importer" nor in MP.
The log show that the files where detected, but were identified as belonging to any other movie already in the database.
In fact, in the "movie manager", "File details", I found the new movies under other different movie... and the option "send to importer" just make them appear under other movie...
I went to "importer settings" to try to change something to solve the problem... and then I saw something that made me think that that was the cause. I deactivated and everything works again.
I had enabled the option "Always group Files in the Same Folder". So I unchecked it and Eureka!