A dirty flame tray. There is either a small tray between the burner tubes or holes at 3 and 9 O'Clock on the burners themselfs that help spread the flame from burner to burner. Pull out all burners and with a paper clip, clean out all the holes. Look at the base where the burners go for a trough. This also must be clean and free of debris. If there is an air shutter, make sure it is most of the way open to supply secondary air to the burner. Once you get all burners to light, you may have to adjust the air shutters to give you the blue flame with tips of orange/yellow. If this does not work, replace all the burners as a set.