Return side flow keeps slowwing down

Just opened the pool and replaced all the sand in my Hayward filter. Pool equipment was running fine with a filter pressure reading of 17 psi and strong flow out of return lines. Next day the pressure reading was up to 30 psi and there was barely any flow from return lines. Back-washed filter and pressure returned to 17 psi with strong flow out of return lines. This morning pressure was back up to 30 psi with barely any flow from returns. Repeated back-wash and with same result ie. good pressure and strong return flow. Can anyone explain what's going on?