You'll need to select which guides get the steam embed in your Prime Settings. Under the Steam Exposure section, add all the guides you want to use the embed on and click the Update button. Also make sure you have the Show Stream In Guides box checked.
Let me know if you've done that and it's still not working.
Let me know if you've done that and it's still not working.

Thanks to Jovy for the sig!
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Any ideas what is happening?