I had a few problems with my immobilizer, make sure the key reader hasnt moved, the metal part on your key needs to touch the metal part on the barrel til the end of the rotation. I think on mine now, it doesn't touch unless you start turning the key, and when you have gotten to to ignition (all the way to so the starter motor is turning) it should still be touching.
I had problems starting my car, sometimes the security light would flash for ages, and all it was was that the reader had moved on the barrel so it wasn't touching the whole rotation.
hope this helps