DIAGNOSIS: This almost always means there's a clog somewhere in the air path. Emptying the bin is good, but hair and debris get packed into tighter spots you can't see.
FIX: 1) Remove the bin and filter. 2) Take a good look down the hole in the vacuum where the bin sits—use a flashlight. 3) Check the main pathway in the cleaner head. Detach the wand from the floor nozzle, and look through the wand and both ends of the nozzle for a wad of hair wrapped around the brushroll or blocking the intake. 4) Clear anything you find. Before reassembling, give the main filter a quick tap over a trash can to dislodge fine dust.
IF THAT DIDN'T WORK: The secondary filter, the one under the bin near the motor, might be completely saturated and need washing. Take it out, rinse it under cold water until it runs clear, and let it air dry for a full 24 hours before putting it back.
HEADS UP: Never run the vacuum without both filters installed and completely dry. A damp filter will choke the motor and feel like zero suction, and running it without a filter will let dust into the motor and kill it.