Swimming pool chlorine 'locked'?

I'm trying to get our pool open as I do every year. But this time I've got a weird, new problem. The water has cleared up (not totally yet) but I can't seem to get a reading at all for 'free chlorine'. i've been shocking it and shocking it and the total chlorine is good (it was 3.4 ppm then i shocked it again with 6 bags of shock, 33,000 gallon inground pool, not sure what it is now).
The pool store guy said the chlorine is 'locked' by something. Anyone know what that means and how I can 'unlock' it?? I put in a salt water converter (hayaward/gold line aqua rite) last year and it's been running on super chlorinate for days. All other levels (ph, stabilizer, alkalinity, etc) are right where they should be. Water temp is around 68 degrees Farenheit.
TIA!
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http://en.allexperts.com/q/Swimming-Pool-Spa-2261/Chlorine-Lock.htm -----
- gpsman
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I wouldn't put another drop of anything in the pool until I got a good water test and see what you have (a good pool store will do it for free) They will tell you if this is a stabilizer problem. Personally I like shocking with liquid chlorine and I am not sure what else is in dry shocker. Generally speaking, less is better when using pool chemicals. Everything you put in there is always going to be there and after a while you can get a chemical war going on. If you don't get a lot of dillution from rain water, draining some out and refilling is the only way to bring down the concentrations.
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