Welcome, and thank you for posting here at the Roku Community, @Picard747!
No worries, as we are here to find you the best resolution possible.
Kindly refer to our Support article: Why is 4K Dolby Vision not working on my Roku streaming player? to help you figure out what is happening to your device, as well as give you assistance to enable this.
We'll be looking forward to your update, as we are more than willing to help you out with this.
Kind regards,
Carly
Carly Y.
Roku Community Moderator