DIAGNOSIS Weak airflow is almost always a clogged filter. The pre-filter might be caked with pet hair/dust, or the main HEPA filter is saturated and needs replacement.
FIX Power off and check the pre-filter. Remove, wash, and dry it as described above. If airflow is still weak, the main HEPA filter is the culprit—replace it even if the reset light isn't on yet.
IF THAT DIDN'T WORK Check for obstructions blocking the top outlet grille or the bottom intake. Also, ensure the unit is on a hard, flat surface; deep carpet can partially block the bottom intake.
HEADS UP Running the unit constantly on high in a very dusty environment can clog the filter well before the 6-month timer. Visual inspection trumps the timer light.