For example, in Return of the Jedi, the first dialogue occurs about 2 minutes and 30 seconds in, after the opening "crawl" has finished. The app has better confidence when syncing to the dialogue and ship/laser effects, as opposed to the long, extended musical cues.
If you find that the app is having trouble syncing to parts of the movie, the most likely reason is that there may be too much music and not enough dialogue for the app to sync with. But as long as the app shows "Listening..." then the app is still attempting to sync and should lock on within a few moments.
We hope to find ways to improve this in future versions.