Boiler has stopped igniting?

My Ferroli F24 has stopped firing up, this also happened last Decemner when 2 parts were replaced. These parts are the parts recommened to be replaced in the trouble shooting bit of the manual, so surely they can't need replacing again! As replacing these bits is the answer given in the trouble shooter I am stumped, what do you do if you have done that and it breaks again, sorry to sound so girly and un- technical, can anyone help,
cheers,
Louise