Possible reasons are:
1. Although the battery voltage is normal, it does not mean that the battery can provide sufficient current. For example, if the battery is undercharged, the battery plates are aging and the electrolyte has a relatively low density, the voltage will be normal but the actual capacity will be insufficient.
2. The mechanical failure of the generator caused too much starting torque, exceeding the starting capacity of the battery.
3. The cable connecting the generator to the battery is too long or the contact resistance at the joint is too great, causing almost all the voltage on the cable to drop, making it impossible to start the generator.