I am playing on an iPhone 6s and recently I was in a highly populated area with lots of people playing. I had Wi-Fi on and GPS was working, but my phone was not showing that I had any nearby. I talked with some young men who told me there were at least 9 nearby. Why wasn't my phone showing them?
Occasionally the servers have a hiccup, and anyone with the app currently open will stop finding pokemon (they also won't be able to access gyms or claim items from pokestops).
Force closing the app and opening it up again resolves the problem. Either you'll see the same pokemon others are seeing, or you'll see the servers are down.