Four-stroke thumpers are notorious for being hard-starters when hot... that's why a lot of the MX racers have a hot-start lever, but they STILL have a hell of a time if they stall on the track.
Hotstart= An air enrichener to help lean out a rich mixture when the engine is hot. (The opposite of a "choke" knob) Great for tempermental large bore singles.
BTW - this is a 4-stroke single problem - my 2-stroke KX250 starts 1st or 2nd kick, hot or cold.