Make sure you have enabled it as a complication in the Watch app on your iPhone.
If you go into there and into complications you should see it listed.
Once that is checked, it might be that the developer only supported one or two types of complication. There are at least 4 and maybe 6 or more types now on watch OS 5; so you might only be able to use it, for instance, as a "Small corner" "Medium Corner" "Modular Large" "Modular Middle" "Text Small" or "Text Large"
If you mean you can only select it as a "card" or enabled on Siri watch face that's different.
If you mean you can only enable it as a complication for the top left, the only complication on the Siri watch face then I believe that is a "Modular Large" and you can use that same complication on the Modular watch face, but not the smaller ones if the developer didn't make them.
I hope this helps or resolves your issue