Airport Extreme deterioration

I live in a 2 story house. My main computer and Airport Extreme are both in the upper floor. When I first bought the airport extreme in April everything worked fine. The laptop and Ipads downstairs had no issues with connectivity. Things have changed. connectivity seems to be limited to the top floor where the equipment is. Downstairs connectivity is gone. What's up with that?

Either the Extreme is genuinely faulty or there's some kind of interference - cordless phones, baby monitors, neighbouring wireless routers

You can check for other wirelessly networks and determine which channels to avoid by using a tool like iStumbler