Im do not know a ton about RC's in particular, but most small engines are reletivly the same. If the Engine sounds like it wants to fire but just wont. Theres a few places to check...
-Glow Plug
-Glow plug Starter (wires/battery)
-Fuel line/Filter
-Possibly Old Fuel?

Or the Engine is flooded. Usually when an engine is close to starting but just wont, in my experiance its flooded. For my DTX.18, I find if its flooded, to NOT Prime it, and just pull like a thousand times, not faster or Harder, but more times...until it sounds alittle different.
Like, when you try to start it with the throttle open alittle bit you can hear the difference VS when you have no throttle on. They may sound like the throttle is open if its flooded.

If its trying to start, usually nothing major is wrong with it. Again, im no RC mechanic, nor Paid mechanic. This is just my experiance.