This can be for many reasons here are some of them:
When the application for the port has been authorised, we would be given a specific date and sometime a time. During this time, it is possible for the number to go offline for up to 24 hours but in most cases, it come back within 2 hours.
Sometimes when a number has left the BT network, for example its moved to Virgin Media this can prevent numbers being ported, we advise you to contact us to check the portability of a number before you start making plans.