When Roku licenses content for inclusion on The Roku Channel, they often are not granted permission to stream all seasons/episodes of TV shows. The content owners decide what they are willing to provide to Roku for what cost. This sounds like Hallmark is retaining most of their content, and just giving Roku users a taste of what's available. You want to watch them all, you have to subscribe to their service.
Just be aware that if you subscribe to a service (such as Hallmark) through Roku, you will only be able to watch the content on a Roku device linked to the same user account that subscribed to it. If you want to watch their content on any other device, such as a computer, you must subscribe directly with the provider.
Dan
Roku Community Streaming Expert
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