Could be flame rectification electrode failing, quite common on these. Are you sure pump is actually circulating correctly or just spinning doing nothing?
Could also be air locked in boiler, have you had any work done on heating system lately?
It is extremely rare for the electrodes to fail but when it does happen you have to change all three. The fault is not measurable or anything visible. Its uaually intermittent. It normally shows a red lockout out light.
Much more likely to be a problem with the gas supply. It needs a competent person to measure the gas inlet pressure when the boiler tries to ignite.
Alternatively it might be a problem with the air pressure switch/venturi.
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