Re: gas stoves.
Admittedly, I am not an expert, but will recount my experience with "bad burners".
Some gas ranges have steel burners and connector, some cast iron. In the case of cast iron, periodically (every 5 years or so) wire brush the the burner holes. If the tubes between the pilots are also cast iron, a long wire brush to open them wider.
On steel , you can also wire brush, but less aggressively.
Now... if individual burners do not light, even when the pilot is going... here's something to look for. Particularly is there is a range hood, over the stove top. Not all of the grease from cooking always goes up the chimney, or into the filter. Over a long period of time, the grease particles can fall into a burner head. Eventually, especially if the burner is not often used, that grease can congeal, go from the burner head into the connecting tube. The connecting tube usually has a small "dip" on the way to the pilot light. If the grease builds up there, it can prevent the gas from going to the burner, even though the burner valve is turned on. There is not enough pressure to let the gas go to the burner head. If the burners are removeable, unscrew the burner and the tube, and poke a flexible wire through the inlet tube, to clear the grease, then user a degreaser to wash out the entire burner. This is a "sneaky" little problem that doesn't show itself, since the grease "plug" is in a semi liquid state.
How did I know this? 'Cuz it happened to me last month at my camp. All is well again. Took about a half hour.