It's not always the case, of course, but whereas with original fic the writer has to create the whole world within the fic, with fanfic there's already going to be so much there, so much established in canon

Ah, that is definitely my problem... I can't *quite* connect to the characters because the world and the character has been built elsewhere and the story is just a little glimpse but maddeningly, tantalisingly just a glimpse...