Are you sure the test kit is good? They do have expiration dates.
The next thing to worry about is whether or not you are doing the tests properly. You need to follow the directions to a T. If it says wait 5 minutes for the color to develop, you have to wait that long. If it says to shakes the nitrate bottle 2 for 30 seconds, you gotta whack and bang that thing for 30 seconds. These drip kits won't provide scientific accuracy, but they get pretty darn close to the real value.