Good nitrate kits are hard to come by. I've had two bad Red Sea kits and now have another bad kit (can't recall the name). To test the kit, use Chuck Gadd's calculator to mix up a 1 gallon stock mix using distilled water. Use that to test the kit. If you added enough to get the tank to 5 and the kit is showing 0, suspect the kit.
I'm currently researching either a Hach or a LaMotte test kit,