1. If you look at your "Instant" queue tab, some of the titles may say "not available on xbox" or something like that under the notes section.
2. When adding new movies to your queue. Make sure you click the separate button "Add to Instant". It will not automatically add movies to both queues by clicking the "Add" button. This messed me up in the past.
However, if a movie in your DVD queue previously didn't have a "Play Instantly" option, but then they make it available at a later date. It will appear at the end of your "Instant" queue automatically.