Wi-Fi not working on some devices?

I have several wireless devices in my home. Lately one of them will not respond to the Wi-Fi and when the password is entered, it says "unable to connect to network". This happens with Ipad 2 and Iphone one will work and the other will not- and the reverse.

I have seen the same thing happen. What I found to be true is that the router's signal gets in traffic with others causing the signal to blink. Its like physical traffic on freeways with cars and trucks. The same holds true with Wi-Fi traffic. You may have two routers, but your neighbors around you may have two or three routers respectively. All of which are on the same signal path. As a result they must hop signals and it takes time to pass the other signal. In terms of the "unable to connect to network" your router might need to be replaced, the built in antenna may be shot.