Turn your pump off; open the lid; put a hose in the strainer basket and fill the basket; have a helper toggle the pump ON once the basket is full. The water should almost instantly be pumped out of the basket.

If not, the problem is the pump, filter, or something else down stream of the filter.

If it is, the problem is between the pool and the pump, on the suction side, OR a leak AT the pump or AT the strainer basket lid.