Kinda sounds like you have some organics in your water which are consuming your chlorine. I would suggest pouring in a couple of 96oz jugs of plain, unscented, 6% Clorox bleach in front of a return while the pump is running, and check the FC level after 30 or 40 minutes That should raise you 12000 gallon pool to at least 7ppm. If it does not you have an infestation that you will need to deal with.