Well, 20 seconds isn't nearly long enough.
Read the directions carefully. You must add the drops from bottle 1, then mix it gently. Bottle two must be VIGOROUSLY SHAKEN FOR 30 SECONDS, then you must add the drops. Once adding the drops, you then must shake the tube for a FULL MINUTE, before leaving it to sit for longer.
I got zero readings for nitrates too when I didn't follow the directions. I would do the test again with the proper procedure to find out again.
The problem with the reagent bottle #2 is that the reactants are powders and have a tendency to settle in the bottom of the bottle and not mix properly. You must follow the directions - or shake the bottle/tube LONGER than it says to get a closer reading to the reality of the situation.