I just opened my pool and noticed that my Hayward DE filter is losing all of its pressure after a few hours of running. I can see air accumulating in the pump lid, but everything appears to be working fine (return jet has expected water flowing. I had a new pool service open the pool for me. Is it possible something was not done correctly allowing air to get in somehow? I checked and cleaned the baskets, nothing visible in the impeller area, performed a backwash and added new DE powder. Anything else I can check before calling them back out?