Title not included in movie set

Hi,

I have a few titles which are not included in their movie sets. I have checked the title with themoviedb.org and it does belong to the movie set, which I also have. I have also tried removing the title from my database, and then adding it again. The problem persists even with fresh new titles added.
An example is the movie Hannibal (2001). I have the other 4 "hannibal" movies in the Hannibal Lecter Collection, but this one is shown in the regular movie list next to the collection item.

1) Why is this happening?
2) How can I prevent this from happening automatically?

Regards,

Q.