Hi,
first of all, you are using different vendors to create an ESSID. That could cause a lot of problems.
I would check if the devices are using different encryptions. A device which connects to the SSID "xyz" with WPA2/AES, maybe wont roam to the SSID "xyz" with WPA/TKIP. Then you can increase the EIRP of the AP to get a better signal strength.
You have to know, that the client decides where to connect. When he decides for his own, you cant really tell him to connect to your AP. You need to "attack" the client from you wifi infrastructure to force him to roam away from unmanaged access points.