When the exhausts are open, The actuators rely on a power feed to them constantly.
Perhaps something to look at would be;
With the engine running, watch the volt gauge and press the switch. If the voltage drops then I'd think gets is power going to the actuators and perhaps there is a mechanical restriction stopping the flaps open. They are known for sticking.
I would also try disconnecting the mechanical flaps from the actuators and see if you can move them by hand (with engine off) if you can, surely it's the motor.
Best of luck....