I can't keep my pool blue!?!?

I live in Florida and have a screen enclosed pool, and I can not keep it blue for the life of me. I add one jug of chlorine in a week and shock it once a week. It will be blue and then like 2 days later the water is pure green again! We run the filter about 8 hours a day. and I try to clean the filter at least once a week. What am I doing wrong?