I know it's frustrating.
My recommendation is that once you're done, keep your FC a bit higher for a while. Not saying to keep maintaining SLAM for ever, just making sure that everything that's still lurking in there gets eventually killed off rather than start multiplying again.
Getting rid of algae means killing it faster than it can reproduce. When there is a very small amount of algae in the water from a few spores that have been been blown in then it takes only a very small amount of chlorine to kill that off. When there is a bit more, then it will take more chlorine to keep that from growing.
The number of chemical reactions that can happen per second basically increases with the number of molecules that are available for the reaction. That's why we SLAM, it makes sure that chlorine is killing algae faster than it can regrow. In the same way the growth of an algae population is faster the more algae there is to start with. The amount of algae basically wants to double in a certain time period.
When there is only a little bit of algae to start with, then doubling that may not even be noticeable, and you'll only need very small amounts of chlorine to kill that. But then there's a lot of algae, then one doubling period can mean the difference between between green side walls in the morning and a full blown swamp in the afternoon, and you need SLAM levels to kill that faster than it can regrow.
With chlorine at "normal" levels, you can control freshly introduced algae material that always comes with the wind.
Once algae has established itself, target FC levels may be able to stop that from escalating as long as you keep adding chlorine. But you notice over time that chlorine demand is unusually high. To actually get rid of it, slamming is needed to not just kill it just as fast as it is regrowing, but faster. You don't want to just kill the regrowth, but what is creating the regrowth.
There may still some algae be lurking somewhere in a protected and hidden spot. Check behind the skimmer weir door, maybe pool lights, things like that. Give it a good brush.
And even then, in the end there might be this tiny amount left in an obscure location without much light where it grows very slowly and lets you graciously pass the first OCLT. But over time it shows itself again. That's where it helps to stay a bit higher after a successful OCLT for a bit longer. Just because you can...