I have an iPhone 6 and its wifi signal is very poor.

When I first bought my iphone that was one year ago, the wifi signal was completely good. But now it's very poor. Before it had all bars, now it only has 2 bars and sometimes 1. Does anyone knows why? I have the iOS 9.2.1. I have done reset settings and the reset network settings. But it still doesn't work. please helpp. Also sometimes when I'm searching for wifi networks, and I'm very close to the router, it doesn't shows up.