I assume you're switching the valve on before you're trying to light it? It's off in the picture.

Too much air backflow could be because of ventilation fans or exhausts elsewhere in the building (bathroom, kitchen, dryer, gas water heater) are sucking air out of the building and drawing replacement air down your flue.

The easiest solution to this is to crack open a window near this appliance, at least to see if it will work. That will relieve the negative pressure in the building. If it does, but closing the window causes the pilot to blow out again, then a fresh air supply will need to be installed to feed this appliance.