Have you tried turning the wifi on and off again? Or rebooting the device?
Only devices that support the wifi roaming standards (802.11k, r, and sometimes v) will automatically switch to the nearest disk as signal strength changes. Otherwise they'll generally connect to the strongest one they see at the point they're turned on. Rebooting that disk would result in the device switching to a disk further away and it won't switch back to the nearer one automatically.
