I am getting low heat / low flame when the valve is turned to high

For propane, improper lighting procedure- Ensure lighting is followed carefully. All gas grill valves must be in the off position when the tank valve is turned on. Turn tank on slowly to allow pressure to equalize. See lighting instructions. For natural gas, undersized supply line - Pipe must be sized to installation code. For both gases, improper pre-heating-Pre-heat grill with both main burners on high for 10 to 15 minutes.