baxi brazillia wall heater ?

I have a baxi brazilia wall heater and when trying to light it ,I get to the stage where you hold the ignition switch in for 20 secs. and then release it to fully ignite. The heater lights okay at the ignition stage but when the switch is released after 20 secs. it fails to ignite, ie. the flame goes out on release.What is causing this problem?