In what player(s) on what device(s)?
What make of TV?
Can you check that this really is an app on the TV, and not on a YouView box?
(Though having said that, Movies is the one part of NowTV that should work on YouView).
Can you use other internet apps on your TV without problems, e.g. BBC iPlayer, and/or Netflix if you have it?
How do you try to watch a movie, step by step, what movie(s), where does it fail and what happens, and what error message do you get, if any?