Your boiler is failing to ignite because the small pilot flame that starts the heating process is not staying lit or will not spark.

Technical description

1. Gas not turned on. 2. Air in gas line. 3. Incorrect pilot lighting procedure. 4. Electrode lead not connected to the rear of the spark igniter. 5. Incorrect spark gap. 6. Current tracking to earth. 7. Pilot injector blocked. 8. Faulty components. 9. No gas to the appliance.