Engine Not Starting

[AeroSpark]

I flew on a 757 out of Gatwick last week. As the engines were being started we saw white smoke/vapour coming out of the back of the No.2 engine, which was followed by a strong smell of fuel in the cabin, so much so that the flight attendant phoned through to find out what was going on. She said nothing to worry about, the engine didn't light first time and the smell was the unburnt fuel which I assume is what I saw coming out of the engine? The engine then started normally and we went on our way. I am curious as to what causes an engine to fail to start and is it a common occurence. First time its happened to an aircraft I've been on but I'm not what you would call a regular flyer.
Thanks for any answers.

[Flightmech]

Most probably an ignition failure. Plenty of fuel flow but nothing to light it. The airplane has two ignition systems so most probably the selected ignition system failed (therefore giving the fuel mist from the exhaust) so the crew motored the engine to purge it of excess fuel and then started normally using the other ignition system. A fairly common occurrence.

[AeroSpark]

Thanks Flightmech

[c100driver]

Somewhat ironic really considering your PPRUNE handle.