Wireless Network

I'm a networking noob. In fact, I hate them which is why I need help! I just finished setting up my network, but I am confused by how the thing is supposed to work.

Three computers connect to the router fine, but my moms computer doesn't. I have to plug her computer directly into the router in order for her net to work. For her computer, I am using a USB wifi adapter, and it's installed fine. When I search for networks though, nothing comes up.

Is there a reason her computer can't connect?