Sorry, let me be a little clearer then. By not pathing, I mean that when they have an open path into the fort, they are not taking it. If your fort is bottled up, it is completely expected behavior for invaders to dawdle around, and I agree with the comments re: clearing a siege to resume trade. However, if they have an open path into your fort or even to friendly units, they should head right over and attack. As far as I know, modders can't control that, its just how invaders work. I have had orc sieges, and they do attack if there is a path into the fort.
The 'bug' I am referring to is that invaders with 'flight' are very buggy and will often refuse to press the attack, sitting at the edge of the map forever, no matter how wide open the fort is. Darkflagrance had this issue with the Harpies civ in Fortress Defense; I don't think he ever found a way to fix it. I think it also happens when invaders arrive on flying mounts (and iirc, it all boils down to whether or not the leader can fly, as units follow their leader). There's just something bugged about the pathfinding when a flyer is involved. (They do appear to pathfind once a friendly unit gets within their physical sight)
I suppose this makes Masterwork a little unique in that it forces you to explore your caverns and even send dwarves deep into them, but having played with Harpies in the past, I know from experience its more annoying to deal with than it is interesting/challenging.
(Incidentally, antmen invade just fine, so its not a caverns issue)