DIAGNOSIS The lower spray arm is likely obstructed or not seated correctly. If it can't spin freely, water pressure drops and cleaning suffers. Food debris or a displaced rack can jam it.
FIX 1) Remove the lower rack. 2) Grasp the lower spray arm and lift it straight up off its hub. Check the hole in the center of the arm and the hub for any debris. 3) Look into the exposed wash pump inlet—shine a light in there and use a chopstick to clear any large chunks. 4) Rinse the spray arm's nozzles under the faucet. 5) Align the arm's center hole with the hub and press down firmly until it clicks into place. It should spin freely by hand.
IF THAT DIDN'T WORK The wash pump might be failing, not providing enough pressure. Also, ensure you're not overloading the dishwasher or placing large items (like a baking sheet) in a way that blocks the arm's rotation.
HEADS UP The filter system needs regular cleaning. A clogged filter reduces water flow to the pump, which then can't power the spray arms effectively. Clean that fine mesh filter every month or so.