We manually vacuum our pool. Lately, when vacuuming the pool, what I am vacuuming seems to go through the filter and is blown back into the pool through the return lines.

When running the filter when we are not vacuuming we have no problem with anything being blown through the return lines and the pool is clear.

Any ideas why this is happening? I have a hayward S310S sand filter and a hayward 1 1/2 HP pump. The pool is about 30,000 gallons.

Thanks.