Ignition failure signals can indicate the following.
No gas to the unit
Broken hot surface ignitor
Bad or dirty flame sensor
loose wire connection at circuit board or plugs
Faulty gas valve
(*Plugged main burner orifices-sometimes a spider will spin webs inside the burner orifice and plug it up)
combution blower motor hose pluged-sometimes garbage can get into the hose and plug it up.