chrismaz15,
I don't know diddly about car engines but I assume they aren't much different than aero engines. Most aero engines, when properly primed and with a good glow plug, can be started with a simple flip of the prop. I don't think vigourously yanking the starter should be necessary.

Perhaps you're not priming enough.
Perhaps you prime too much and flood the engine, in which case forcefully turning it over may damage it.
Check your glow plug.