I was flying from Dubai to Heathrow and was preparing to descend, i put 16000ft into the Altitude selector and waited till T/D. as soon as i got to T/D i clicked on hold so that the place would descend but it didnt. It just stayed at 36000ft. I tried using the V/S Selector then pressing Altitude hold again but every time i did that the plane would just level off. Whats going on