I have a poor link from one ExtremeN router to another in the house. They barely have a line of sight, but the distant router shows as a pretty poor link. Do these routers have a favored direction in which their ability to broadcast and receive is stronger?
Whats inbetween these routers? In other words, whats in the walls? Things like concrete walls will hurt a signal among other things. Wireless is very funny with it comes to a signal. I've seen things that are outside hurt a signal before and its things that are out of your control.
Allegedly they have multiple aerials, so they should be pretty much omnidirectional. Like he said, check for any weird walls, Faraday cages, metal staircases, etc, between one and the other.