I am not a Basecamp expert, I do a lot of cussing when I use it. But you might try checking your Options. Under Edit \ Options then Activity Profile and Routing. Check Fastest vs Shortest and if any of the Avoidances are checked.
If that isn't it, you may want to check your version level. BC 4.2.1 has been released.
Hope that helps. If not try cussing at it. It doesn't help me but I feel better afterwards. :mrgreen: