Buz23

I successfully used PSCAN to read the EKA on my MG zs 180, 05 reg). I noted it down closed off the software, got out of the car, locked it with the fob, removed the fob from my keyring and moved it well away from the car. I then used the EKA, put the key in the ignition but car still immobilised. To get the car started I then had to lock it with the fob, then unlock it with the fob and it started.
Any ideas ? Am I missing something ?