I'm attempting to auto-route a fairly large board file in Allegro PCB design/Specctra. Some parts are already routed, but there are at least 3 layers with no etch at all that the auto-router can use, as well as many other layers with only some etch on them. However, when I run the auto-router, I get a large number of "Unconnections", which never seem to route even after hundreds of route and clean passes. I don't understand why this would happen, because if the auto-router will allow clearence conflicts and crosses at the beginning of routing, what would cause some wires to be unconnects throughout the entire process? I can look at the wires and find that most of them are easily routable by hand without moving anything, so why does Specctra find them so difficult?