Deleted songs still showing

Apologies if this is an FAQ but v grateful for any help: when I do a full rescan, mt-daapd usually removes from the database songs which have been deleted from the media location. But for some reason it doesn’t catch all of them – I have half a dozen “ghost” songs in the db, which have been deleted but which firefly swears are still there. I can of course delete them manually, but I’m just wondering if there is a way to make firefly realise that they’re gone. Anyone?