This is a common problem with newer generators that are not run on a regular basis. Closer tolerances within the fuel systems to meet today's emission requirements make fuel systems more susceptible to air effecting start up. This is not as common with older generators. Older generators that experience this problem may have a leak in a line or check valves that are not properly holding the fuel in the engine.
Lighter low sulfur fuel has lower flash temperature, which causes the block heater to flash off some of the fuel within the injectors. One small bubble of air within a unit injector can cause an injector not to fire at start up. If enough injectors do not fire, the engine will not start. This failure is 100 percent preventable by periodically running the engine during weekly inspections. The engine does not need to be run until the coolant temp comes up to normal operating temp. All that is needed is enough time to verify that the engine will start, that the air is cleared from the fuel system, and that the generator comes up to voltage and frequency. This can be completed in less than five minutes. Any additional test running would simply burn up fuel and air quality maintenance run time.