You might have your water tested for phosphates. If they are 3000ppm+ as I suspect, it will consume the chlorine quite rapidly. I've been there and have to treat every 3 years or so. Not many people have this problem, but phosphates build up in my pool and I find it impossible to keep chlorine in the water more than a few hours when I let them go. Sounds like that's what you have going on to me.....I use ALUM when it gets that high, kinda messy...have to vacuum to waste after treating but it's effective.

Hope this helps, Tony