@hanna2212
I don't think the removal, restart, reinstall procedure will work for you but you will have to cancel the subscription first before being provided the option to remove. For a subscription through Roku, just login to your online dashboard at https://www.roku.com/ (there is a little user account icon in upper right of screen). Then go to your subscriptions.
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I haven't subscribed to Britbox through Roku before, but others have also reported this issue with missing content before. I have subscribed to Britbox through Prime and also directly with Britbox before and haven't seen similar issues.
By subscribing with Britbox directly or through Prime, allows you the extra benefit to watch on both Roku and non-Roku devices.
I believe it is Britbox's responsibility to keep their library content updated even when used through Roku Channel (since it really is just a payment method), but that means Britbox has to interact with their developed app and update the content.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
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