The API nitrate test kit is the only one of all API tests that can be problematic! You need a relative new kit for it to test reliably. I have used already 5 different API nitrate test kits. One went bad on me within 2 month. I haven't figured out yet where the problem is.
Every time I am testing, I use standard test solutions first, so I know when there is something wrong. Some people here have suggested to vigorously shake both bottles for quite some time before testing, but that seems to be only part of the problem. As I learned, some nitrate kits go flat rather quickly and they cannot be reactivated.
Incidentally, I have had nitrate levels in tanks as high as 200 ppm, but even at that level I never observed any problems.