Have you tried temporarily removing the thermal cut-out from the circuit to see if ignites then? It's the little resistor looking thing inside the plastic sleeve/cover that runs to thermostat. Disconnect it from the thermostat terminal, and from wire that is on the other end, and you should be left with a female spade connector at end of that wire. Connect that directly to thermostat terminal where the other part was connected, and try it. If it works, the thermal cut out is no good and needs to be replaced for safety reasons, in case of flame out of control, it blows and shuts gas valve off.