Automatic routing is still an NP complete problem, as in there is no perfect solution. This is the same issue with automatic routers for PCB's and the like, it

Automatic routing is still an NP complete problem, as in there is no perfect solution. This is the same issue with automatic routers for PCB's and the like, it is guess work along with some genetic sort of algorithm that steps through various options. I've had times where the auto router for a board gave up because it couldn't complete the routing, ripped everything up and did it by hand. Sure it took longer but the routes were shorter, and I finished routing the board with room to spare.