Before adjusting the levels of chlorine, you must first test for and adjust the pH of the water to be within the stated range for your setup, usually 7. 2-7. 4. When the pH is out of range, the ability of the chlorine to either stay in solution (dissolved) or remaining in its' active state, is badly hampered.
I would add a lot of liquid shock to the pool to give it a quick boost in chlorine. If that doesn't work I would add a chemical called conditioner and stabilizer. This will bring up your cyanuric acid level. When this level is low chlorine will have no life and will dissipate quickly.
There are several things that could cause your chlorine level to be low in your pool. Do you have a lot of leaves in your pool. This organic matter will cause you to the chlorine to burn off too fast. Try trimming back your trees if you have this problem.