When you "prime" the generator for starting, you should hear the fuel pump. Remember it is not starting to prime until after the light comes on, so you want to HOLD IN the prime button for about 5 to 10 seconds "AFTER" the light turns on. To prime, press in and hold the opposite portion of the start switch, either the start switch inside your PC, or the one on the generator itself.
If your generator fuel pump is working but may be weak, try opening and then closing the main fuel tank gas cap to relieve potential negative pressure inside the main fuel tank. Negative pressure requires a strong good operating generator fuel pump.
As stated by others, make sure your main fuel tank has enough gas in it. The one time our generator ran out out gas, our E350 gas gauge showed the generator cut-off was closer to 1/3, not 1/4. Maybe we were parked on a slope when it happened just that one time for us.