"Chlorine Lock" is one of those terms used in the swimming pool industry that is not widely known by the general public. It describes a state in your pool water where the free available chlorine that is present is unable to do its job as a sanitizer. If the chlorine cannot sanitize then it also cannot burn off organic matter that enters the pool water.
Generally there is only one reason for chlorine lock - Cyanuric acid levels over 100ppm.
Maintain your Cyanuric Acid level at 40-50ppm. Cyanuric Acid is generally known as Stabilizer.
The quickest and easiest way to solve this issue is draining the pool in the same method as you would for high TDS, 12 inches at a time, testing each time, until you reach the desired CA level or between 40-50ppm.
If you do not control the reduction of the CA then be prepared for your pool to "turn". Without the chlorine able to act as a sanitizer your pool will suffer quickly with algae blooms.
Bob Charlton is a Certified Pool & Spa Operator and Inspector with the National Swimming Pool Foundation with 15 years of experience of pools in Florida.