Low Time,
I think Brian and bb in ca and the others have it right. A slight loss of prime will make the engine delay a bit on start. I would think you can hold the starter a bit longer than 6 seconds, and you should see it light off. There is no harm in waiting 10 or even 20 seconds, although the fact that it lights immediately after an aborted 6 second crank tells me that it will probably light before 10 sec. If you have fuel flow gauges, note the kick as the gas finally arrives in the engine as the sign to expect lightoff. If not, note the fuel pressure light blinks as the venturi pump wets itself.
If the lightoff delay is an ignition failure while fuel is flowing, the fuel flow gauge and a white fog exhaust are the signs (and of course the quiet in the head set instead of the slight tick-tick of the igniters).