I added a wiring diagram in the image below so that you can see all of the components connected in series between the timer and the gas valve assembly. The ignitor must have 110-120 volts applied to it or it will not glow. In order to diagnose the failed component will require an Ohm meter to test each component for continuity.
There could be one of many possible component failures preventing your ignitor from glowing starting at the timer:
Timer (it must close switch #2)
High limit thermostat (key#34 must have continuity)
Operating thermostat (key#41 must have continuity)
Thermal fuse (key#39 must have continuity) Check this one first for continuity.
Flame sensor (key#9 must have continuity)
Motor centrifugal switch (5M&6M must close once the motor has started to complete the neutral supply to the ignitor)
Remove the rear panel to access the high limit, operating thermostat and thermal fuse. Check the continuity of each one using an Ohm meter. Disconnect one wire from the component before checking the continuity of the component. Which ever component is found not having continuity has failed and it will need to be replaced.
NOTE: Disconnect the power cord from the power supply before removing the back panel.