The filter is clogged. A saturated HEPA filter creates too much resistance for the fan.
Fix: Replace the True HEPA filter. The pre-filter may also need cleaning or replacement if it's loaded with hair and large dust. Ensure the new filter is installed correctly, seated flat with no gaps.
If that didn't work, check for physical obstructions. Remove the grill and inspect the fan blade for damage or debris wrapped around the spindle. A damaged fan blade will not move air efficiently. Also, ensure the speed selector is fully engaging the Turbo setting; a faulty switch could prevent the fan from reaching high speed.
Heads up: Weak airflow on a new filter indicates fan motor failure. The motor may be running but not achieving proper RPM. This requires professional repair.