In many cases, the simplest reason for high chlorine levels in a pool is because too much chlorine is being added. Chlorine works by binding with microorganisms in a pool's water, thereby dissipating itself. If chlorine is added too often and while monitored levels are much above 3 parts per million (ppm), it may just sit there because it has nothing to do because there isn't any bacteria for pool chlorine to attack.
In outdoor swimming pools, one of chlorine's greatest enemies is sunlight. When exposed to a lot of sunlight, chlorine will dissipate rapidly. Keeping a pool covered with a dark pool cover can be a cause related to stubbornly high chlorine levels. With no sunlight hitting a pool's water often enough, any added chlorine may just sit there, too. Combine frequent chlorination and a lack of consistent sunlight and see how easily chlorine can remain persistently high.
Often, the most effective quick fix to the problem of stubbornly high chlorine levels in a pool is to use a chlorine neutralizer. Sodium thiosulfate is the chemical of choice to reduce chlorine in a pool down to recommended ranges. The typical chlorine reduction dose of sodium thiosulfate is about 6 oz. per 10,000 gallons of pool water. When used to lower pool chlorine levels, sodium thiosulfate is often applied when those levels start exceeding about 6 ppm.
The recommended level of pool chlorine in a pool's water is 2 ppm. Chlorine shouldn't be allowed to decline below 1 ppm or go above 3 ppm. One of the simplest ways to reduce chlorine in a pool is to just not add any more until it finally dissipates. Test for chlorine levels about twice weekly and also monitor pH balance. Pool water pH that's between 7 and 8 won't usually have a problem with high chlorine levels.
Maintaining effective pool chemical balances is made much easier by using a good all-purpose pool test kit. At minimum, use a testing kit to ensure that chlorine levels are where they need to be and that pH balance exists. Also, monitor your pool's chlorine stabilizer level. Cyanuric acid (CA) will help prevent sunlight from eliminating chlorine and will allow it to be adjusted with little risk of over-chlorination.