I know we're beating the issue a bit but... I find the the AP nitrate kit is particular to the little things often skipped. Shake the bottles well before use. Add the 1st bottle, mix, add the second bottle, mix again, let sit 5 minutes. Any shortcuts will often give a false negative.
The other possibility is if the kit is old. Depending on where you bought it it might have sat on the shelf, or is this your's from your previous tanks? On the top of the label for each bottle is a lot number. The last four digits are the month and year of production. I believe the nitrate test is supposed to be good for three years.