I think Niantic Labs (the developer of game) has disabled the nearby Pokemon tracking option. You should install a third-party app called PokeVision or another one called Poke Radar. These are the third-perty apps that can help you to track, find and catch more Pokemons nearby your area.
Poke Radar: http://www.pokemonradargo.com/
Alternatively, you can also use this. I think this one is the easier of them all. https://github.com/mchristopher/PokemonGo-DesktopMap/releases
Works great from home post-patch and allows you to create a temporary mobile website of the scanner you run out of your computer. Not sure how well it works when you're walking around outside, though