When you add CYA in the Spring, I would wait a few days and possibly more like a week before you test for it. It takes that long to get an accurate reading. Could you have been testing too soon after you add?

Secondly, It seems some pools drop CYA over the winter and others will maintain it.

Last, If you have algae...you need chlorine. Post some current test results and folks here will help send that algae to an early grave.