That cursed algae returns.

About three weeks ago the pool water turned green. A test strip indicated that chlorine level was ideal.
PH, Alkalinity and stabilizer readings were border line.
I took a water sample to the pool shop who sold me a packet of stabilizer, a bottle of green algaecide and
Alkalinity UP. As the water was green I could not use the stabilizer until the water became blue.
I then used the algaecide and waited about two weeks while the algaecide did its work, added a cup of chlorine granules every second day. I then added the stabilizer, then the alkalinity Up over a period of a few days
while adding some acid to lower the PH.
Now, about a week down the line, the water is turning green again. A test strip gave the same results as last time. Last night I added three cups of chlorine but it has had no effect.
My filter sand was replaced 14 months ago and I cleaned out the filter two months ago.
Short of repeating the entire process again, starting with another bottle of algaecide, is there anything I am doing or not doing to cause this problem? Some expert advice would be welcome.