Well if the immobiliser was on the car wouldn't start, it has been working, and preventing the car from starting during scenario's when the car has been left unlocked for a certain period of time.
Am i safe in assuming that you think that the immobiliser is activating, but just not working? if so, i have no idea how you would actually tell that it is on or not, there are no dashboard LED's or warning lights, it either works... or it doesn't
from the owners handbook
NOTE: The engine will automatically be
immobilised 3 seconds after the starter switch
is turned off, and immediately when the starter
rey is removed from the switch.
and
ENGINE IMMOBILISATION
Engine immobilisation is an important aspect of
the security system and is designed to
safeguard the vehicle from theft, should the
driver forget to lock the doors. Engine
immobilisation prevents the engine from being
started unless a valid key is inserted into the
starter switch, and occurs automatically
whenever any of the following conditions
occurs:
•Three seconds after the starter switch has
been turned off.
•As soon as the key is removed from the
starter switch.
The engine is re-mobilised by a signal to the
starter switch transmitted from a transponder
contained within the key head. This occurs
automatically whenever a valid key is inserted
into the starter switch and turned to position ‘I’.
how do you know it's not being imobilised ? - there is no specific switch - it is activated by the immobiliser in the ignition switch reading a signal from the keyfob - as soon as you place the fob near the ignition or the key in it's re-mobilised.
you say 'if the immobiliser was on it wouldn't start' - not with someone elses key it wouldn't or if someone smashed the lock & tried to hot-wire it , it wouldn't. But as soon as you put YOUR genuine key/fob in/near the ignition it re-mobilises it.
Don't understand WHY you think it's not working