Mine did/does the same thing.
Make sure the air vent in the gas cap is clear.
Try to start the engine with the gas cap off and see if that helps.
Remove the plug and cycle the starter many,many times to clear the engine block of old oil and stale gas with the valve shut off If you see a fog or spray come out of the spark plug hole,that is the problem.
Make sure the choke plunger in the carb is shutting off.
Set the top throttle stop carb screw to 2 1/2 turns out from being bottomed out. The 2Nd screw air mixture down to 1 1/8 turns out.
Make sure the carb float needle is not leaking past the seat and flooding the intake. Read my post on how to clean it.
Don't rule out a bad spark plug. Unless it is new,replace it. The spark from a new plug is much stronger.
I'm replacing my carb as it always seems to run very rich after the engine is warm and if you shut it down when hot is very hard to restart.
The carbs are old and have had alcohol gas left in them which attacks the aluminum and the brass tube seats at the aluminum holes.
I turn the gas off and run the engine until it stalls out when I'm done riding it which seems to help.
With today's blends of gas from state to state the octane rating will be effected by the ratio of oil to gas you use. Many of the 2-stroke sites say to only run high octane gas.
I've asked with no reply as to what the fuel/oil ratio should be for these engines. If your engine is too oil rich,it will not start as it wets the plug with oil.