ZonePlanner probably didn't take into account the concrete walls for the 2 distant users, but... does the iPhone6 even work correctly when it's closer to your single AP?
From your ZD's Monitor/Access Points page, if you click the first (blue/green) icon to the right of your AP, it will save the System Info (support text) file.
Search the file for the iPhone6 client MAC address, and client statistics will show RSSI and PER. You want PER < 10%, and RSSI - PER >= 25 for a "good" connection.