If Ihave an engine that doesn't start -

#1 Is it getting fuel? If Idrip a drop into the cylinder does it fire? And, while Ihave the glow-plug out . . .

#2 Is the plug hot?

If it kicks after that and dies - is the carb jet blocked? Sometimes crud or a piece of plastic or fuel tube gets dragged in. Clear the jet.

If those are both yes and it still doesn't try to catch

#3 Is the fuel good? Try fresh or borrow a drop or twofrom someone&repeat #1

As noted, leaks in air cause problems, but only after the engine is running.