Problem with key fob and immobiliser

[Medved]

Hello, I would really appreciate some advice here. I have two keys for my Skoda Octavia Mk1. One works perfectly, but the other has suddenly stopped de-activating the immobiliser. The central locking action works fine, but when I attempt to start the car, the light on the dash flashes to indicate that the car is still immobilised. Can this be caused by a weakening battery in the fob or do I need to reprogram it in some way? If so, can anyone explain how this can be done? Thanks very much for your help in advance.

[Eddie-NL]

have you dropped the key recently?

[Medved]

I don't believe so.