Many factors go into answering this question. Some possible reasons are;
1. Dirty filter or baskets (skimmer & strainer)
2. not running the filter & pump long enough each day
3. Chemical problem
Lets discuss each.
If you have a sand filter, the sand may need to be replaced or just try getting the proper pool chemical and clean the sand first. Sand should be cleaned several times a season and changed every 3-5 years (this could vary depending on pool and chemical usage). If you have a cartridge filter, make sure you have a clean cartridge. If you have a DE filter, make sure you have enough DE Powder and that the grids are clean. Every time you backwash your DE filter, you should add the recommended amount of DE Power.
A lot of customers only turn their pump on at night. With high heat and humidity, the pool water will always look cloudy. If you have a continuous use pump, your filter system can run 24/7 during the summer. the more you filter the water, the cleaner it is going to be. With a chlorine generator, make sure the unit is in the right function to produce the proper amount of chlorine from salt.
Water chemistry is the most common reason for cloudy water. All these chemicals have to be in the proper range or cloudy water could be the result;
Chlorine reading - 1.5-3.0
pH - 7.2-7.6
Alkalinity - 120-140
Calcium Hardness - 250-450
Stabilizer - 30-50
Please note that based on the chemical system you are using, these recommended readings may vary. If you bring a sample of your pool water to your local Medallion dealer, they can test it and give you the advice you need to eliminate cloudy water.