What is wrong with my Ferroli Modena 102?

I'm having a pilot flame problem with my boiler. For the last month I have noticed that the pilot light and flame does not come on when I switch on the hot water tap or the central heating. However, after switching on the hot water tap or the central heating, if I hit the boiler on the side this seems to cause the pilot light and flame to ignite. Also, i've noticed that if the central heating is on and I switch the hot water tap on and then off, then the flame goes off even though the the central heating is still on. Then, I have to hit the boiler again to re-ignite it. I have already changed circuit board, thermostat and water pressure switch but still get the same problem. Any ideas??

I am having to tap my boiler to make the pilot light come on so that I can get hot water or for the central heating to work. I have changed the circuit board, the thermostat and the water pressure switch but this has not changed anything. What do you recomend I do?