We have an 18X36 vinyl lined pool. Each year when I open I have this issue for 6-7 years. I will shock it and next day there will not be any chlorine level. I opened pool this year(which was crystal clear when cover pulled off) and put in 3 bags shock. Next day no chlorine. Next day I put one pound in and was 5ppm 1 1/2 hour later no chlorine. Could this be chlorine lock? I am told I have nitrates which I am told are caused by fertilizer or decomposing animals. I have never found any dead animals just some worms about 30 and leaves. As a reference my neighbor has just as many worms, dead voles snakes etc and no issue. I have a solid fence with little greenery around the pool. I checked pitch of concrete and rainwater flows away from the pool. I had a mesh cover and switched to solid with no help. What I do each season is drain about 10-12 inches below skimmer 4-6 times and refill. Then the pool will act normal for the rest of the year , but when I open the following year the issues come back. Any ideas, I bought the nitrate test strips and I dont get much of a reading on them so not sure if that is the real issue, I dont have confidence in poolm store. Is there a lab I can send my water to to get a definative answer.