Some Pokemon Go players are becoming frustrated with the game’s Nearby Tracker, which is supposed to show which Pokemon are available nearby. Ideally, the Tracker would show every single Pokemon that’s available in the immediate region. However, the way it actually functions is by checking all the PokeStops within a certain distance (possibly half a mile), and displaying nine of the Pokemon at these stops. While fans are still trying to figure out what criteria it uses to decide whether to display a Pokemon on the Tracker, it appears to be entirely random.
So if a rare Pokemon spawns near you, it may be up to luck whether you even see it on the Tracker before it despawns.
Issues with the Tracker have plagued the game since its release, and many fansites have been created to bypass the Tracker entirely. However, Niantic continues to work to improve the Tracker’s functionality.