my powermax 155 will not ignite?

Radiators were not heating all over so decided to flush them out. Turned off gas/electricity/water(header tank). Drained water from boiler/radiators. Put Sentinel X800 jetflo into header tank, and turned on water/gas/electricity supply. Turned on boiler, mains light comes on as does the fan, but green ignition light or boiler does not. What have I done wrong??.