In general, the appearance of citations are affected by three things:
- The metadata included in the citation.
- The type of reference.
- The Citation Style chosen and the way it uses the metadata field for each type of reference.
It sounds like the style you have chosen does not use the Full Journal field for some reference types. If you have some citations looking right and others wrong the best approach is to change the reference type or metadata. If you only want to make this change for a single document (and not in your library), you can go to Paperpile --> View all references in the Google Docs menu and edit the references there. Note, however, that if you do this, that citation will no longer be linked to your library and will no reflect any changes you make to the library copy.
It is also possible that the problematic citation has been unlinked by a previous edit and therefore the document-specific copy of the metadata does not have the Full Journal field which you added to the library copy.