I have two WAPs in my house with the same SSID, different channels, far apart from each other to offer coverage.
When I physically move my laptop, while on, from one to the other it loses the wifi connection. I have to turn wifi off then back on to get it to work again.
I have other devices, iphone and Android, which manage to work seamlessly across the house. The only problem is with the Mac laptop. Also, it used to work. This might be just a problem since upgrading to Yosemite, but I'm not sure.