Please help!!! The pool guy said it was normal but it doesn’t look normal at all. As soon as it’s on it works great and pressure is good for about 15 minutes. Then slowly air bubbles start to form inside the pump skimmer until it becomes one big air bubble and no water movement in the pump skimmer. And returns are down to almost no pressure. I backwashed and changed a few o-rings. I have a new DE filter just purchased last season. Any tips please?