Check your battery voltage.
These little generators work on a voltage in / voltage out basis. If you give them 12.5 V DC in but they will give you 125 V AC output. While they can clip higher voltage they cannot boost lower voltage. To prevent damage to electronics the generator just shuts down if the voltage is too low. While 9 volts will start the engine, it won’t make safe AC voltage. As long as you are holding the start button down, as mentioned above, the generators safety switches are bypassed, once you release the start button the safety switches are part of the circuit and do their job.