Rich I've had a similar thing happen twice. Once on a burner in oven, the other time on our forced air heater. With the oven it was the valve. It was not opening properly. Gas would be released, but not enough to bring the mixture to ignition percentages. If you are getting a strong flow through the intake damper that means the combustion chamber is breathing. That is good. The orifice where gas flows onto the HSI may be sooted. You can have gas flowing through the burners and up the flue but never reaching the ignitor. This is fairly common, especially if you have had low flame issues in the past.
On the heater it was the HSI not being hot enough. It was glowing, but it was about to go out. If you have ever touched the thing with your fingers that cause problems. This is really rare from what I've been told.
Do you have a multimeter?
