I have a similar problem with a bit of a work around.. I'm not sure if it works for others and I'm also trying to understand it myself because it is a bit annoying. My laptop will always see my wifi network but sometimes my wife's MacAir or my moms Samsung tablet will suddenly not be able to see the network. By freak luck I discovered that turning on and then off the MAC filter fixes this problem. From my laptop, I type in (192.168.1.1) to access my modem and under the wireless settings, I activate and immediately deactivate the MAC filter and then hit save. (If I wait more than 3 seconds to deactivate the MAC filter, then my laptop will not connect and I have to use another device to access the modem settings so be sure to do it quickly.) When I hit save, all devices will be able to see the network. But this only lasts until the next day, especially with my mom's tablet. With my wife's it can last months before having to do it. It's a workaround for me but pretty annoying.