Sounds like restricted fuel flow for some reason.
Did you check everything mentioned above? Do you get gas to flow out of the petcock when you apply vacuum to the hose?
Is the tank clean inside? There are screens on the petcock that can clog, and an inline filter at the carb inlet as well.
Fuel filters -
https://faq.ninja250.org/wiki/How_do...fuel_filter%3F
Petcock info -
https://faq.ninja250.org/wiki/Rebuil...tcock/fuel_tap