The upper and lower spray bars are not normally powered by anything other than the pressure of the water coming out of the angled holes at speed IIRC. Might be worth removing the arm and running water through it to clean it through.
Had the same thing a while ago. strip as much you can from the bottom arm and clean it all out. Mine was the same. Arm wouldn't spin. I removed a part to clean it, couldn't remember how it went back in so binned it! Works fine now, better than before.
