The spark electrode is also used as means of flame proofing......the main flame must engulf the electrode to produce a milivolt signal to circuit board that shows main flame is lit.
No signal and circuit board will attempt to lite 3 times and then lockout.
IF you have main flame then adjust flame so it engulfs electrode.
Also check that ground wires are clean & tight.
IF you do NOT get main flame then check for obstructions in burner tube, orifice and for power at gas valve solenoid(s) during sparking. Power is only at gas valve solenoid for 6-8 seconds during lighting attempt(s).
Otherwise.....circuit board may be faulty