I'm typing this for DH -since he won't post LOL!!
make sure the lower knob is not connected to the follower in the back. Move the dial all the way to the right, and loosen (turn counterclockwise) the screw right below the point of the dial. This will increase the pressure a bit and should keep it from moving to zigzag when you don't want it too.
My DH has the same setup. He said there is no positive stop on this, so it can move if you bump it. The bottom knob is only for engaging the backpack.