What could be the cause for your pool whips to stop moving after two hours of filtration when the pressure reads normal on the sand filter?

Clean and empty in ground pool filter, clean and empty filter near pool pump. Be sure all whips are clear of dibris. also do a flush of the system. If this does not help, you will need to have a C02 charge done between the pool line and the pool pump. OR you may need to have your impeller replaced in the pool motor.