If the igniter is weak or has the wrong resistance it may take too long to glow red and be the wrong temp and color which can interrupt the ignition timing. After 4-5 trials for ignition it will flash a lockout code and probably start the cycle after a 1 hr timeout period. If the gas valve is not being energized same scenario. Is their gas turned on to the house? Had a call once where goofs turned the gas off as a prank.