What's that? Drain it and start from scratch?Put another 10ppm of CYA in with the sock method. Test as often as you can, up to once an hour and keep getting back up to 12ppm of FC.
Low CYA would explain a rapid drop off of FC. But if you keep the FC up by testing and adding often, too low of CYA wouldn’t stop it from killing algae too.Crud, no wonder there hasn't been any improvement.