Wow, that's a huge loss in a very short period of time. For the test, you can use 10ml, with one scoop of powder, and then each drop will account for 0.5ppm of FC.
Where are you storing the chlorine jugs? Are they outside by chance? I had a bottle stored outside, for just 2 weeks ... and in the Texas heat, it significantly reduced the potency of it.
You may need to start off with adding 2 gallons, then test 20 mins later ... if it's down to 10ppm, add another gallon, and then repeat until you get some movement.
If it is ammonia causing the issue, like DorsalSpine mentioned ... Its out of my experience for dealing with it in any other way that hitting it with SLAM levels of chlorine. Your challenge will be maintaining the level during the 1st day, and being able to keep it at the 20ppm until you break down enough of it.