When a reference does not update with new metadata it is usually because the document-specific copy of the reference has been unlinked from the library copy. When this is the case, go to Paperpile --> View all references, in the Google Docs menu (or Manage references, in the sidebar menu). This will open a page to the document-specific copies where you can edit the metadata there.
Note that editing a reference on that page is generally the act which unlinks the reference, so in general avoid editing references there that are working.