Well, unless you have a HUGE house, I would expect most wifi routers to be able to reach all parts.
It might be worth looking at Apples's Airport Extreme to replace your router, and using an Airport Express as a signal booster. As a bonus, you could connect wireless speakers to the Express.
I know this doesn't help, and I understand the frustration when one device works and another does not. I have one of those teenager things that lives in the basement and only surfaces when Netflix stops working, so I'm well used to this problem.
I recently changed my network security from WEP to WPA2, because my son's iPad mini refused to connect, after months of working flawlessly. No problems since then. It could be worth trying that out.