We replaced the sand in our filter this season. When starting the filter, it starts with lots of pressure into the pool through the return. After 45 minutes the pressure decreases so it doesn't filter properly or have enough suction to vacuum. Turning the pump motor off then on increases the pressure again but we lose it over 45 mins or so. Have tightened all fittings, checked 'O' rings, cleaned pump impeller. What could be causing this loss of pressure?