Yep, try shaking and banging both nitrate bottles for 30 seconds (bang them a few times against your palm or on a hard surface to break up the reagent powder inside). When you add them to the tube, make sure you're adding them in the right order and the right amount of drops. Then shake the tube vigorously for 60 seconds and then wait 5 minutes for the reading. With that said if you just did a water change of if you have light stock or a lot of live plants you might not be showing much nitrate, but try what I said above and see if that helps.
