It's definitely possible to get a false low reading on your nitrate, especially if (a) you haven't tested in a while, or (b) you have never fully shaken the second bottle of reagent. the reagent is a powder and drops out of the solution quite quickly. Despite the instructions saying shake for 30 seconds, in my experience you need to shake for a couple of minutes, banging the bottle on a hard surface a couple of times if necessary. It's possible the powder has all but solidified in the bottom of the bottle if it's been sitting around for a while.
The low readings will show whether you have a reading of zero or anything above, if the reagent is not mixed into the solution.