Computer Help and Support

They are radio waves, metal and other devices that radiate radio frequencies can be the culprits.
This link is a good primer "What Is A Wireless “Dead Zone” And How To Eliminate It?"
One thing that we often forget is the WIFI signal is finite. Location, location, location is not only a mantra for real estate but also router placement in the house. Our house is small but the WIFI is weak outside. The house was sheathed in foil covered foam board with steel doors so the only outlet for the signal outside of the house is through the windows. Fortuantely we only have double glazed windows and not Low E glass which effectively blocks WIFI as well.

http://www.makeuseof.com/tag/what-is-a-wireless-dead-zone-and-how-to-eliminate-it/