I think you have either running the motor too rich, running too cold of a plug, or a combination of both.

A. The glow plug has to get hot enough to ignite the fuel. If the glow plug is too cold for your model or if there is excessive fuel (aka rich condition), this problem can happen.

I would work on a) making sure you are running the correct glow plug and b) lean the high speed needle by 1/2 turn and trying again.

Try this and I bet it will work better.