Biasi 24S - erratic supply of hot water?

Every time I turn on the shower or any tap for hot water, the ignition goes on but would then switch off and the boiler would then attempt to ignite again, but without much success. After about what seems like ten million attempts, the flame becomes permanent and the supply of hot water becomes more steady. I've switched off the boiler, I've pressed the red button that's on the right hand side of the switch, but not sure what else to do.

Help!