Many things can cause WiFi interference. Other networks in the area, microwaves, some phones and Bluetooth(this should not be too bad anymore, but may still happen).
You could try to change wireless channels on your router/access point.
If you have long enough cord length, try to move the router closer to the room with poor signal.
If that fails, wifi extenders just get placed in the farthest area of good signal and repeat it(thus you get more signal strength at a further distance.).
Remember to get one that supports the same standards as your router. This is important to get the highest throughput out of it.
So if you have a 802.11 A(5ghz)B(2.4 ghz)G(2.4 ghz)N(2.4ghz and 5ghz depending on the router) or AC(2.4ghz and 5ghz)