This has just manifested in the last few weeks on All The Tropes -- while existing talk and forum pages retain full Flow functionality, new talk pages open in basic wiki format.
Using Special:ManageWiki/namespaces, you can change the default content model of the talk namespaces to Flow. This should force new pages to be in the flow format. To migrate pages, they will have to be deleted, or moved to an archive location. Unfortunately, there is not currently a way for this to be done automatically, although I believe there should.