wrong size cone insert,I carry 3 or 4 different size inserts in the flight box for different size spinners,none of my spinners have groves,if the engine is over primed or flooded the starter has a hard time turning the engine over.I normally put the cone against the spinner,but if I'm having trouble turning the engine over,I'll start the started first then shove it against the spinner, also as in the other post, make sure the battery is strong and has a full charge,mine is a close cell from the lhs and will crank g23,120 size engines with ease.
Have a goodun,John.