Do you have a timer on your pump?

If you added CYA, and then cycled your pump on/off before the CYA was fully dissolved (a week+ with cold water!), odds are the CYA you added is sitting on the bottom of your filter tank.

When you turn your pump off, the DE mostly drops to the bottom of the tank. When you turn your pump back on, most of the DE gets stirred back up and re-coats the filter membranes. But, that would NOT happen with the CYA granules! They are too large to 'float' back up, and would have remained on the bottom.