When i put my central heating on my boiler fires up for about 5 minutes then stops.?

when it stops i leave the oil on to see if it will fire up again but it doesnt, i have to then turn it off again and leave it for a couple of hours before i can try it again, if i tried it straight away it wouldnt even fire up. when i do leave it for a while and try it again the same thing happens, it only stays on for about 5 minutes. what could be wrong with it?