I have a Magnasweep in floor pool cleaner. It's about 11 years old, had ol style acrylic balls controlling watrer valve. I have gone through all pressure check, water valve, gears, etc. are operational. All zones work but some heads pop up minimally. Several zones pop up minimally, but when I purposefully pop them down, that zone will pop up full tilt with max pressure....like it's supposed to. When I remove those heads to check, they appear to rotate normally. Is this likely faulty heads or old water valve not working correctly?