I am a new pool owner. I normally have my pump running about 12 hrs/day and I vacuum usually at least once a week, keep chemicals balanced, etc. It seems that just when I vacuum it and it looks very nice, I get up in the morning after the pump has been running all night and there is some dirt that has settled to the bottom, usually right in the middle of the pool. What can I do differently that will get all of the dirt up? Thanks.