Please note the mod above improves the airflow, however some of these RTA's have a manufacturing fault where the inner chamber is press fitted too low into the outer chamber, either restricting the air flow or even cutting it off completely.
This can be resolved fairly easily by lightly screwing in a 3mm tap into the inner chamber, and then using a 2mm punch, gently tapping against the tap from the mouthpiece side ... this will free the inner chamber, where you install a 3mm * 1mm O-ring and re-insert it