My apologies. I think it supports the paid-for Showtime, not Showtime Anytime (the one used for existing subscribers). That would definitely explain this issue.
But not all the Showtime apps. Just the one you have to subscribe to through Apple. The regular showtime app that you can access if you're paying through your cable provider doesn't have siri access.
It supports both showtime apps and the same goes for both HBO apps. I use showtime anytime and HBO go and it works with it. If it isn't working for you you should try restarting your Apple TV or contact Apple for trouble shooting assistance.
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