Where did my chlorine go?

Hi all and thank you for hosting this forum. I have been looking for a good pool forum for many a year. We have a 30-year-old Anthony plaster pool. In great shape after all these years. Only to us it seems like 5 years but you know how that goes. Well to out current problem. For the first time this year, we can not keep a chlorine reading. We have double, triple and quadrupled shocked. We finally put in 22 lbs. of crushed up tablets that our pool store recommended. Still no reading. Thinking about turning the pool into a nice vegtible garden. Hehe! Just kidding. Help please. Thank you.