Flourish Nitrogen Plant Food?

Hi, I recently started using flourish nitrogen plant food as the plants in
my aquarium stopped growing due to the nitrate level dropping to 0ppm. On
the label it says that it contains two sources of nitrogen, one being from
nitrate and that you can use a nitrate test kit to tell how much nitrogen is
in the tank by doubling the reading. But I am still unable to get a reading
using my Hagen test kit, even when I add the plant food directly into the
test tube?

Does anyone know why that is?

cheers
Evil Hand