Could be a lot of things, but start with the easy things to check.

Is your air filter clean? Is the intake restricted in anyway? A dirty or clogged filter will cause warm starting issues.

Does it smell of fuel when it won't start hot? Take the spark plug out and have a look for a 'wet' appearance straight after a failed attempt at starting. How good/new is your spark plug?

When did it start doing this? Can you think of anything you changed, modified or did differently at the time?

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