Apparently, SWCG's are sized with the assumption that you'll have 60+ ppm of CYA in your pool.

Get some dichlor (sodium dichloroisocyanurate) -- almost all the quick dissolve shock at places like Walmart is this stuff -- and use IT to get your chlorine up, and keep it up UNTIL your CYA is more than 60.

Then see if your SWCG will maintain.

PoolDoc.