Harrier A/P

I am playing with A/P settings on a number of aircraft, as its a handy way of learning the other aspects of flying.

I downloaded the Harrier recently - and its superb.

For some reason, I can't seem to get the aircraft to go (and remain) on the correct altitude setting, but have no problems with heading or speed settings.

It seems the altitude preset is on a slightly different panel from the other settings, which includes the 'master' switch. Am I missing an 'activate' switch for altitude?

Once I get all 3 sorted, I can experiment with hover, etc.