The reason for this may be related to internet connection. Movie rentals, even though downloaded, will require an internet connection to activate the countdown for the 24 hours expiration period. This might be the reason why it did not play.
You may need to validate the rental prior to watching so it starts the rental period timer and for that you will need an internet connection. You can probably do that by starting the rented movie while connected on a Wifi or right before leaving home. Once you have started playing the movie rental, internet connection is no longer required. You can play anywhere without internet connection.
For more information on movie rentals, you can visit:
https://support.apple.com/en-us/HT201611
Note: 24 hours time limit is for United States customer only, while 48 hours for other countries.
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