every month my pool goes green

Aloha- I been servicing and cleaning pools in Hawaii for 30 years. I take pride in keeping a crystal clear pool. My latest pool is a 24,000 gallon, plaster lined, chlorine /sand filtered pool. I keep having to treat it once a month with Yellow treat and super chlorinate, then it will stay nice for about three weeks and then repeat the process. I've been keeping the chlorine high, brushing, vacuuming and backwashing regularly. My sand was just changed out 2 years ago and looks good. The return lines have plenty of pressure to them so I do not believe it to be a circulation problem. Our local Pool doc and I are baffled?
Any ideas?