AT&T Samsung Galaxy S6 will not connect to WiFi automatically

I have an Asus TM AC-1900 WiFi router at home. We have a Nexus 4, Nexus 5, Nexus 7, iPhone 6 Plus and notebook computers. They are all able to connect to Wifi when I get home. However, the new AT&T Samsung Galaxy S6 will randomly fail to connect to WiFi when I get home. It does manage to automatically connect some of the time but most of the time I have to remember to connect it manually. Is this a defect on my particular unit? I have tried doing a factory reset but it doesn't seem to help.