ignition problem?

hi anyone out there,
Having a problem with hot water/ heating water boiler ingnition,upon start fans runs greater speed than normal "tick over" speed as should, but boiler does not ignite,have changed both h/w and ch/water sensors,have cleaned fan, have blown out silicon tubes to pressure switch,blown out p
press/switch.
If i blow down short silicon pipe from press/switch (short blow) boiler ingites on cent/heating.
If i blow down same pipe (bigger blow) hot/water will ingite.
I presume this should be the normal duty of the fan.

any suggestions please.........