Just purchased a Bestway 18x9x48 above ground pool from costco with a sand filter this past weekend. I followed all the backwash and rinsing instructions. When the Control valve is set to circulate the pressure is at 6ps with a good amount of flow going to the pool. When the control valve is set to filter the pressure is 1psi or less and the flow is very low. I removed all the sand and cleaned the Screen then reassemble without adding any sand. The psi is still low and so is the flow. Does anyone have any suggestions what I should do?