Why doesn't my Worcester Junior 24i fire up immediately?

Boiler is 5 years old and serviced yearly. Lately it does not fire on demand CH or HW. Fan comes on as does pump and I can hear the ticking from the ignition. On demand there is a loudish twang/buzz noise. Ignition stops and the process (Fan, pump, twang, ignition) starts again. Usually 3-5 attempts and then burners come on.