UPDATE
Found the solution: the A, E and R switches have to be ON. The manual says they are normally ON, but this is not true; by default they are OFF and you have to turn them on in order to make the pitch and bank switches work.

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Has anyone run into the following problem:

In autopilot, clicking/holding the PITCH down/up button has the same effect: it brings the nose up, whether I click the down or up side. There is no way of bringing the nose down. However, the height button functions correctly: once an altitude has been selected, it brings the nose up or down as needed to regain the altitude if the plane has dropped or climbed.

Any solution?