Hi and welcome back from the Dark Side!
We need a set of measurements to answer it. Otherwise it's just a guess.
We'd like to see:
FC
CC
pH
CYA (stabilizer)
and
TA
The relationship of CYA to FC (free chlorine) is what I'd guess is happening, but high pH can reduce chlorine effectiveness as well.
Again, I'm guessing you don't have a high enough CYA level and UV rays are breaking down your chlorine. Still, in this horribly hot summer across the nation, using less than half a gallon of 6% in 14000 gallons isn't bad. I'm using almost double that in 20,000 gallons.
I actually think you're in good shape.
Carl
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