You have to understand what happening in the pool chemistry and specifically what happens to Chlorine in the presence of Cyanauric Acid, a.k.a. stabilizer a.k.a. CYA. In lieu of understanding the chemistry, at least you have to trust the chemical science behind it.
For a pool to stay alagae free, using chlorine as the main algaecide, then the ratio of FC to CYA needs to stay roughly, 1:10 ratio 1 chlorine for every 10 CYA. (teh science is more exact than 1:10, but this is an easy number to remember).
If you are using powder shock, and chlorine pucks, those things contain CYA, and so your CYA increases a bit every time is used in the pool. However the Chlorine levels do not increase proportionately, and so after a period of time the FC/CYA ratio cannot be maintained, and it doesnt take long, and before you know it, you have algae.
My CYA is 42. But according to the ABC's my FC should be pretty high. I have never been able to keep my FC as high as it says to keep it. Even with my Auto Chlorinator full blast it will only go to about 4ppm. I think it is because I live in SE Texas and it stays 100+ degrees around here all the time. Plus the recommendations of FC/CYA vary greatly between pool professionals. I am being told 7% of your CYA is plenty. That only 3ppm or so.