Answer:

It likely has to do with either the free chlorine level - using the chlorine up faster than you are putting it in (make sure you test and adjust the free chlorine 2 to 3 times a week and add shock dosage as necessary - any time free chlorine reaches 0 ppm) or the circulation. You may have to direct the return jet to get proper circulation. Make sure the water is moving everywhere in the swimming pool. If you have areas of slime, this is likely the reason. If it is the whole pool, lack of free chlorine is probably it.