In App Purchase for DTS

Sorry you are running into trouble here.

The recently released Dolby and DTS in-app unlocks are free for Pro users and follow the same App Store purchase method as the main Infuse Pro license, so normally if you are able to activate Infuse Pro then there shouldn’t be any issue activating these free purchases with the same account.

However, people use Infuse in many different ways, so if you are running into issues activating these for yourself or family members, you may consider using Trakt method to share purchases. In some ways this is a simpler option and can help avoid issues in certain cases.

More info can be found here.