Diagnosis
This is almost always a fuel/airflow issue. You have insufficient draft or low-quality charcoal. The most common cause is charcoal that is too large or packed too densely.
How to Fix It
First, confirm you are using lump charcoal, not briquettes. Ensure your charcoal pieces are fist-sized or smaller. Do not pack the firebox tightly. Leave air gaps. Use a minimum of two natural fire starters placed under the charcoal, not on top. Light them and leave the dome open for 10 full minutes to establish a strong coal bed before closing the dome and adjusting vents.
If That Didn't Work
Check for a hidden obstruction. Remove the grill grates, heat deflectors, and firebox. Look for charcoal fines or ash completely blocking the lower draft door or the holes in the firebox. Vacuum it out thoroughly.
Heads Up
The dome thermometer can be inaccurate. For critical cooks, always use a reliable digital probe at grill level to verify temperature. A 50-degree variance from the dome thermometer is common.