I first came across this hot start issue with my Gilbern Invader. Starting was a bit of a lottery until I obtained the correct starter - but - although it would usually start from cold, it would never start hot, until after about 20 minutes to cool off.
The reason is that when stopped, the hot water migrates to the top of the engine, and the cylinders add to the heat, so the pistons tighten up. So although the common view is that cold starting is harder than hot starting, that's not the case.
