I changed my pool filter sand yesterday in my Hayward sand dollar SD40... I did everything correctly, got everything hooked back up...my pentair 1.5 hp pump will recirculate the water but once switched over to filter It stops working.. the pump just runs but no water goes into the pool from the filter return and all pressure is lost in the filter... SO, I took everything back apart, opened up my filter..the water is to the top and  looks normal..my pump basket was cleaned yesterday and put back together, so what could possibly be wrong..