Chlorine Demand - Par Pool & Spa has a good handle on how to treat this growing issue.  If you haven't dealt with it yet, you will.

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Watch the Video on SHOCKING

Follow Up Question:
Thank you for your prompt response-I plan to call my local store (NY) to check on the chlorine demand testing-In an effort to do anything other than buy & add up to 50 lbs. of any product I did call another store who recommended using 3 bags of non-chlorine shock-potassium monopersulfate to break the chlorine lock-Does BioGuard have a product like that?

Thought I'd give that approach a try-How can you tell if the lack of chlorine reading is due to a lock or demand? Might be less expensive to drain a third of the pool and refill???  

Follow Up Response:
3 bags of monopersulfate (BioGuard Oxysheen) are not going to do anything.  8 to 10 will have some effect.  When you're trying to take care of a chlorine demand / chloramine problem, you can't play with it.  Trying to save money is only going to cost you more, very soon.  Not to mention a continuing problem all season long. That's why the chlorine demand test is so important.

A partial drain & fill will help, BUT then you have all of that rebalancing of the water to do.  AND it's not going to rectify the problem.  The problem will continue to worsen as the summer goes on & you want to use the pool.

Our local chlorine demand testing is showing superb results.  Customers aren't necessarily happy with the cost, but their problems are done & over with.
 

Question:
I am having a chlorine problem with my 19,000 inground vinyl liner pool.  My combined chlorine is 10 and my free chlorine is 0.  My PH is 7.7 with Total Alkalinity 128.  My pool dealer says to add 35 bags of shock.  I understand I need to achieve breakpoint chlorination (your website has some good info, by the way).  Is there a more economical way to do this? 
What if I wait a few days- will the Combined Chlorine do down?  And if I do super chlorinate, do you recommend regular chlorine shock or the chlorine free?  How long do you think it will be until the chlorine levels return to a normal and safe swimming level?

Response:
If you were my local customer, I would first have you use 10 - 12 lbs. of a non-chlorine shock to help bring down the combined chlorine.  I would be afraid that the 35 lbs might not be enough & would make the situation worse.  Hopefully your dealer has a chlorine demand station to give you an accurate test (look for a BioGuard dealer with the new Accu- Demand 30 test station, it's 99.99% accurate).  After adding the non-chlorine shock, such as BioGuard Oxysheen, retest it after 24 hours.  When it's under 4.0, then do a chlorine shock.

Unfortunately, there's no easy way to deal with this situation.  The combined chlorine will not go away; if anything it will worsen. You're on the right track!

Chlorine Demands can also be attributed to Bio-film problems. Here's some information.