changed my VW key fob case but the immobiliser as kicked in?

[eastlondon02]

I replaced my VW key fob case with a new fob as the key ring clip broke, I I just moved the actual key and brain to the case, It worked it unlocked the car but moment I started the car the immobiliser kicked in... I don't understand why as its the same contants as the old key.. The same key and remote just been moved to the new case nothing else...