Hi Damien ,To know whats going wrong I would need to see a debug log after you have a new movie added to your library and next download occurs (next restart or configured time). You can also delete a few movie entries from the download cache at C:\ProgramData\Team MediaPortal\MediaPortal\Trailers\myvideos.xml, then restart MediaPortal and show me log after it tries to re-build the missing entries from your collection and download trailers.
i will first try to delete some of the movies from cache file ,and see what going on ... will do tonight .