Poor starting
Ask the engineers to check the number of cylinders that the Primer is feeding. If the aircraft starts well when primed with the throttle then it is possible that not all the cylinders are primed using the priming system.
If the aircraft primer is only primeing one or two cylinders that could be your problem. As to priming using the throttle, we start all our aircraft that way - it prevents over priming from too much enthusiasm. If an engine is primed when the engine is hot and rich you will have the possibibilty of a fire whatever.