Interesting.
How often do you test FC and add chlorine? Daily? Twice per week? Perhaps the FC is getting low enough between doses for algae to start to grow, and then your doses up to FC=6 just wasn't enough to beat it back into submission. And every time that happened, the algae gained ground.
How many hours a day does your pump run? If it is a two-speed pump, is it running at high speed at any time other than vacuuming?
What is the current pressure in your sand filter, and when did you backwash last? Perhaps the filter is simply clogged, dramatically reducing the amount of circulation?
Do your return jet have an eyeball that you can aim? If so, aiming it to one side can create a gentle circular motion to the water, ensuring even chlorination throughout the pool.
Here's a random one: When you test FC, are you perchance using a 25ml sample, but using the 0.5 multiplier from the 10ml sample size? If this were the case, your true FC would be less than half of your measured FC.
Yeah, too many questions.
Just trying to cover all the obvious bases.