Joyce,
Below is a cut and paste about something Ben wrote previously about potassium peroxymonosulfate:

"Your pool store is creating a problem . . . and then selling you more stuff to fix the problem they've just created. To be fair, they probably don't know that they are creating the problem. Potassium monopersulfate tests as "combined chlorine". So, when your chlorine is low, and you add MPS (monopersulfate), you appear to have the very "combined chlorine" you were trying to get rid of! Glory be! Better add some more MPS; you've got the dreaded "chlorine lock" . . . whatever that is. (It varies from store to store, and region to region.) "

Get some plain, unscented bleach and add enough to get your chlorine up to around 8ppm. Continue to add bleach each evening until your pool can hold chlorine. Don't add any more products right now except bleach.

(If you want to read more, go to www.poolsolutions.com and use the search there to search for potassium peroxymonosulfate. You will have to be logged out (until your registration is completed) to be able to read the search results.