Not all showtime content is accessible through the Roku Showtime App
Not all content that is available on showtime is actually available through the ROKU showtime app. For example, the series 'Let the Right Ones In' cannot be viewed through the ROKU subscription right now. It was available for some time but has been removed. Since the subscription to showtime through ROKU does not include access to the showtime apps, I have no way of viewing this series.
Many channels on Roku are maintained by the channel provider themselves. Contents from the specific channels are solely managed by their support. We indeed recommend contacting Showtime Support to report the issue and get help.