Just replaced my filter and pump. On first start up pressure reads 19 with good return flow and good suction on both skimmer and side wall suction lines. After a few days the discharge and suction flow slow down but the pressure on the gauge still reads 19. I shut down the pump to check pump basket and impellar for obstructions and fill the chlorinator. Impellar and basket were both clear, primed and started pump to again great flow after 1 day or so flow seems to start to slow down again. What could be causing this? Thanks in advance for your help!