It's possible that the CYA is being backwashed out of the filter before it has fully dissolved. We normally recommend that you start with a clean filter, then add CYA via the skimmer and wait a good week before backwashing, if that's necessary. Or add CYA using the sock-in-front-of-a-return method which doesn't get affected by backwashing.

Until you can get a testable CYA level your SWCG is working too hard. I'd turn it off and go with straight bleach or dichlor powder for a while.