Just talked to my dealer. He checked my phosphate level and it's zero. He says I should be getting 4 months from each cartridge, but I'm getting 6 weeks.
He said that the granular shock (orange top bottle) that I regularly use contains a stabilizer and my levels of stabilizer were off the charts. There's a name for it, some kind of acid, I think it begins with an "a". Anyway, he sold me some liquid chlorine and told me to drain the tub and start over with fresh water, and to get rid of that shock I've been using. We'll see how that goes.