These units are prone to bad timers or bad connections at the timers check your connections and wiring from your timers down to your ignitor and that complete circuit check your voltage going down to your ignitor should be 120v and test the unit for 20-30 minutes and your ignitor ohms should be 0-50 ohms. Check your tech sheet for the wires you need to test. Pop the port open in the front to see if your ignitor is glowing. If it's glowing every single time and it's not releasing gas you may have a gas valve faulty also.
http://www.youtube.com/watch?v=Q8j6fQM3P34