Sometimes you get a bug or spider that crawls into the little air intake around the bottom of the plenum. Also, make sure you are pointing the little knob up when lighting a cold stove and don't open the valve too much when trying to light a cold stove, just cracking it will usually do. The generator needs to be heated up by the flame of the burner to properly vaporize the fuel before you can crank it up. Take the tank off outside and open the valve and see what kind of atomization you are getting out of the nozzle. They sometimes get rust or dirt in them. It can be removed and cleaned.
If that's not it, go to a camping/outdoor store. They usually have rebuild kits/replacement generators for sale.