alarm on sprinter 330x1

hi i have  a 2009 sprinter just had a clifford alarm fitted  it set  to my van key fob,   when i press button to lock van its ment  to chirp and set indicaters of  but it doesnt it chirps twice  when i unlock van   the only way  to set alarm is press lock button press unlock button then press lock button again  then a bout a min later it sets itself,,

when alarm is set if i open doors one  at a time the alarrm works   the bloke who fitted alarm said one of the sensors is not locking door ,   but surely if that was the case why when set i open doors alarm works  any idears thanks