Can you see it when it lights? If you can, make sure the flame sensor is in the flame. Murphys law states that when an orfice or burner port is to be plugged up it is to be the one directly under the flame sensor.
also, is this unit hard wired or is it a plug in unit? Either way a good ground connection is a must for these flame sensors to work properly. I have seen them work without one and, in fact, mine is one of them. However I recently fixed one that worked intermitently and the owner changed out almost every component before he called me. He was running his heater off of an extension cord that someone gut the ground prong off of. We swapped cords and it fired right up.