Fuel Hose and Regulator: The output of a propane tank or your natural gas line is much greater than you need for grilling so the regulator controls the amount of fuel that can flow to your grill. The regulator attaches to the tank (or natural gas line) by a flexible hose with an O-ring to create an airtight seal. Regulators are preset by the manufacturer and should not be adjusted by you. If you look at your regulator you will notice a small vent hole in the center. Common problems here are clogged vent holes, which can cause irregular fuel flow and can lead to trouble. Usually you can clear it by tapping or blowing into the vent. Other problems are fuel leakage caused by a worn or damaged hose or O-ring. To determine if there is leaking mix dish soap and water in equal parts and coat everything from the tank to the control valves. The tank needs to be connected and on but the control valves off. If you find a leak replace that part.
Control Valves: The controls do just that, control the flow of fuel to the burner. Each burner on your grill is going to have a control valve. The valve consists of several components that are specifically designed on a number of factors. You cannot repair a bad control valve and if need be, you should replace the whole unit. Before you do, however remove the control valve from your grill and inspect it. Like other parts of your grill, insects love to climb in here and make their homes. At the heart of this is the orifice. The orifice controls the flow of fuel and can become clogged. If it is, use a thin wire to clean out. Make sure you put it all back together the way you found it. Without the orifice you cannot control the amount of gas flowing to the burner and run the risk of explosion.
Venturi Tubes: The venturi tubes connect the control valve to the burner(s) and mix the fuel with air to provide flame. To mix the air into the fuel there is an open gap in the fuel line here that can easily become obstructed. Insects, especially spiders, love this space and given half a chance will move in as quickly as possible. The best solution for this is to wrap the venturi tubes with an aluminum screen that will not block the airflow but will keep the critters out. These days many grills come with protected venturi tubes. Another common problem here is a misalignment of the venturi tubes with the burner. Typically the venturi tubes are simply placed in the fuel line and can get knocked out of place. The venturi tubes typically have adjustable shutters. You may need to adjust these to regulate fuel flow.
Burners: Burners come in many shapes, sizes and materials particular to your specific grill. What the burner is made of will tell you pretty much how long it will last. Burners range from aluminized steel at the low-end to cast brass and stainless steel on the high end. Low end burners will typically last about 3 years under normal circumstances. Because the burner is inside the grill it tends to get coated in burnt grease and can corrode quickly. Inspect and clean your burner regularly to avoid problems. If the burner is damaged or too heavily corroded you will need to replace it. Get the same size and shape of burner but consider purchasing one of a better quality metal if possible.
Barrier: In between the burner and the cooking grate is something. I call it a barrier; some people call it a radiant. It is called a radiant because it is supposed to absorb and release heat evenly to the cooking surface. However I don't find it to be very good at this. The way I see it the barrier protects the burners from drippings and creates a place for grease to collect and burn off. Either way what I'm talking about is lava rocks, ceramic briquettes or metal plates. These need to be replaced periodically as they become crusted in burnt grease and food and can create an unpleasant flavor on foods as they age. Lava rocks, because they are porous tend to need replacement more often. Metal plates can typically be cleaned and used for a longer time. Inspect your barrier. If it is broken up, heavily coated, or simply not creating a sufficient barrier, consider replacing it.