not sur about citroen but with the fords and vauxhalls, you dont just key the number in.
if code is say 3456, you would hit key 1 three times, key 2 four times, key 3 five times and so on, once the correct code is on the screen, you press and hold one of the other keys until a beep is heard. on vauxhall i think it is the as button.