Generally, if your engine doesn't have transition problems, you probably have the low speed mixture pretty close to right. If the engine is brand new, it may not tolerate low RPM very well until it is broken in well enough. Check your glow plug first and make sure it is not fouled out. A lean run can destroy a glow plug pretty quick. If the glow plug is not lighting up, you may be flooding the engine with fuel. The starter may simply not have the power to turn over a flooded engine (nor should you try). Does the engine start when you flip it by hand (using a chicken stick, or broom handle, or anything to protect your hand)?