In order to answer why, these are some basic questions that must be answered.
1. If you have a sand filter, when was the last time you either cleaned or changed the sand?
2. If you use chlorine tablets, do you use the 4" or 1" size?
3. How many hours a day do you run your pump?
4. What is the level of Stabilizer in your pool water?
If you have a sand filter, you should clean the sand at least once each season and change the sand about every three to five years as long as you are cleaning the sand each season. Oils from your body, sweat, suntan lotion all get inside your filter and make the sand less effective. You need a chemical to break up these containment and make your filter more effective.
With the use of chlorine tablets, if you use the 3" size, they many not be dissolving quickly enough to meet your pool's chlorine demand. You may also have a very low stabilizer reading. Chlorine tablets have stabilizer built into them but they need to dissolve quickly enough in order for the sabilizer level in your pool to be above 40. If below that, the sun will take out the chlorine almost as soon as you get it into the pool.
In the very hot summer months, your pump needs to run 24 hours a day 7 days a week. You can't run the pump only at night or only during the day and keep the proper chlorine level in your pool. You will spend more money on chemicals to control algae and acid levels than you will with energy. A good test kit is a must. Try the five way test strips. Please keep in mind that this is just a generic overview of a few of the main causes of low chlorine problems. Before adding any chemicals to your pool be sure to have your water properly tested then read and follow the instructions printed on the labels. Always remember "Safety First" when dealing with chemicals.