I recently installed a brand new IntelliflowXF VSF Pump. However, my GPM (Gallons per minute) keeps dropping for a given RPM (Speed). Eg. I set this pump at 2500 RPM and had over 40 GPM yesterday, Today, it has dropped below 10 GPM. Is this normal? Note that I have a relatively clean filter (Hayward DE4820). Any points on how I can maintain my GPM would be appreciated.