DIAGNOSIS Spinning spray arms need clear nozzles and unobstructed movement. If they're stuck, it's usually because a tall item (like a utensil or tall cup) has popped up through the rack and is physically blocking the arm's rotation.
FIX Open the door mid-cycle (it'll pause) and visually check all spray arms. Manually spin each one. If an arm is blocked, remove the offending item from the rack. Also, check that the lower rack wheels are fully on the track and the rack isn't sagging and hitting the arms.
IF THAT DIDN'T WORK Pop off the spray arms (they usually twist or clip on) and look inside the mounting posts for food debris. Also, run a toothpick or pin through all the spray holes to clear any mineral buildup or bits of plastic from packaging that might have been left inside.
HEADS UP Forcing a spray arm that's being blocked can crack the plastic mount or strip the gear in the motor assembly. Always clear the blockage before trying to manually spin it hard.