Accuracy of Taylor 2006 kit with high Chlorine water

I shocked my 300 gallon spa with 12 oz of liquid chlorine 8.25%. This was a week following it's first fill of the season after I added an initial dose 2 oz of MPS. Only used spa once since and that was after having taken a shower, but then found combined cl was 1! Now my dpd test reading is 4 ppm for free cl and still 1 for CC. I then tested with the cheap HTH drop test and the color was orange which is what I would have expected after the addition or 12 oz bleach.
FC is probably 20ppm, so why is my dpd test showing only 4? If that is incorrect maybe my cc readings are wrong also?

Are your reagents old? Some of them break down over time, although I don't know if the DPD powder is one of those. In any event, all I can add is that I've tested my water with well over 20ppm FC in it several times after shocking and received expected results from my 2006.

Try using an extra scoop or two of dpd powder?