Just a quick guess here, but from your description it sounds like the sail switch is not proving air flow is good (even though you feel air blowing for that first 20 seconds) to the control electronics. If the electronics does not receive the air flow confirmation it will not open the gas valve and continue with the flame start process. If the sail switch is bad or not free to move when the blower first starts blowing (after thermostat calls for heat) then you need to check that first I suspect.
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