This can be cause by several things. Taking it from the fuel source to the flame, lets start with the burner.
Newer propane tanks contain a self regulating feature that will automatically slow down the flow of fuel if it thinks it's too high. This can be solved by turning the tank valve off completely and disconnecting it. Open the control valves (on the grill control panel) and ten close them. Reconnect the fuel tank and slowly turn on the valve. Now light the grill to see if that fixes it.
Otherwise check the control valve(s) and venturi tubes for obstruction and alignment. Make sure that the flow of fuel is continuous.
Check the burner for clogged gas holes. You can usually see this problem by observing how the grill burns. If there are spots without flame then you probably have a clogged burner.
Uneven Heating: One side or one area of the grill is much cooler than the rest.
This problem is similar to the low flame problem. Check there for solutions. If you have multiple burners determine if one burner is burning hotter than the other.
Yellow or orange flame: The flame should be blue with yellow tips.
Try cleaning the burner or letting it burn on high for 15 minutes.
Venturi tubes may not be properly aligned.
Venturi shutters might need adjustment. Do this by first locating the venturi tube adjustment screw. This screw releases the shutters. Light the grill and turn to low. Loosen the screw and open the shutters until the flame is mostly blue. Turn off gas and tighten the adjustment screw. Let the grill cool.
Grill produces a lot of smoke
There is typically a large build up of grease in your grill. Give it a good cleaning and preheat for 15 minutes to burn off any residue.
Flame from under the grill, behind the control panel
This is caused by misaligned or blocked venturi tubes. After the grill is cooled, inspect to make sure all parts are fitting together. Make sure to check hoses and fuel lines to make sure they are not cracked, melted or burned through.
