Because viz.COLLIDE_BEGIN_EVENT only calls onCollide once per collision, you get one notification of collision when the avatar hits the city model's ground, even though the avatar moves on to collide with the city model's buildings.
so events go like this: avatar drops into world -> collide begin event called -> avatar is still colliding with world but walking around -> avatar walks into building but never lost touch with city model so no new collide begin event.
I think the better way to move you avatar around in the world is with a velocity motor created with the male.addVelocity fuction. There are a few tricks on how to use a velocity motor, so check out the docs on "addVelocity" and the example script in this article:
http://vizworkshop.com/articles/walkthrough_app