My kit instructs ONE heaping scoop of powder. Does my one heaping equal your Two regular?
Here's a link with pics for the CYA test, you may find it helpful...
Ok. Lets see. You are adding chlorine, enough to raise you to 12. How soon after adding are you testing? One hour? And it's to 3 or 4? What kind of product are you shocking with? If it's bleach, do you trust it's strength? What brand is it? Possible it's lost it's strength and your not ever reaching 12? Maybe try a different brand or strength and see if that makes a difference? I'm going to
guess that you are killing the algae as fast as you add bleach, and you aren't adding enough to reach shock level. Try testing after 30 minutes, and add more to get back to 12....? Guessing here....
If the drop is during the day, the sunlight will deplete it rapidly which tells me you don't have enough CYA, not that you have more....

this is perplexing isn't it.... so perhaps add more CYA.
You definetly have a grasp on the process, I can tell by your posts, so it may be something wrong with the chlorine source, or just needs some POP (pool owner patience) in there too? Shock, POP, Shock....LOL sorry wish I had the magic bullet for you. Hope this helps.