Document IDs not activating on "child" sites

Hello:

We have turned on Document IDs on the main team site of our SPO instances, and they are functioning as desired on that site. (Most specifically, we need the "durable links" functionality, so that when a file moves from folder to folder, a shared link to it remains valid.)

This is working as expected in the Document Library on the main team site.

However, document IDs have not been generated on other sites we have created within the instance. (We use such sites primarily for file-sharing with key clients. etc.)

We followed these instruction, which seem to state that the Document IDs feature would propagate throughout the whole SharePoint tenant. 

https://support.office.com/en-us/article/activate-and-configure-document-ids-in-a-site-collection-66345c77-f079-4104-ac7a-e25826849306

Is there some step that we are missing?

Thanks