DIAGNOSIS This is almost always an issue with the engine not running at the correct speed (60 Hz = 3600 RPM). Low voltage means the engine is lugging, usually because it's overloaded or the governor can't keep up.
FIX First, turn everything off and disconnect all loads from the generator. Start it up and let it run for a minute. Use a multimeter or the generator's own display (if it has one) to check the output voltage and frequency at the outlets. With no load, it should be very close to 120V and 60Hz. If it is, the problem was overloading. Plug in your appliances one at a time, starting with the largest, to see which one causes the voltage to sag.
IF THAT DIDN'T WORK If the voltage is still low with no load, the engine's governor or carburetor might need adjustment. There's an idle speed screw and a governor adjustment screw. This is a delicate process; turning the wrong screw the wrong way can cause runaway RPMs. It's often best to refer to a service manual or consider professional help for internal adjustments.
HEADS UP Consistently running appliances on low voltage can damage their motors. If you're not comfortable with carburetor adjustments, this is a good point to take it to a small engine shop.