The API nitrate tester has 2 bottles. The instructions say to shake bottle #2 before use, and to shake the tube after the drops are added before starting the timer. This shaking is important as one of the reagents in bottle #2 settles on the bottom of the bottle and the shaking is to redistribute it throughout the liquid. I understand that some other makes also require one bottle to be shaken before use.
Failure to shake gives false readings. It is generally recommended that a new bottle or one that hasn't been used for a while should be tapped on the worktop first to break up any solid lumps. And shaking the bottle should be longer than the instructions say.