I'm having trouble with altitude-hold while in autonomous mode. The system doesn't respond to a drop in altitude.

The system recognizes when altitude increases: the servo rotates the props upward to bring blimpudino back down.  However, when altitude drops, the servo doesn't react at all; it just keeps the props in the "straight-ahead" position.

The servo should point the props downward to bring the blimp back up to the starting altitude.

There's no mechanical binding with the servo or the props (I have full range-of-motion when in RC mode).

When I monitor the altitude via the FTDI cable, changes in altitude (both increases and decreases) are recognized by the ultrasonic sensor.

Where else should I look? 

Thanks all!