Wireless clients can't connect when I extend my network with an AirPort Express

I'd like to create a roaming network so that all corners of my building have good WiFi coverage.

Here's my setup:

• Mac Mini running OS X Server (with DNS, DHCP, Open Directory enabled)


• 24-port switch


• Airport Extreme Acting as our router (connected to cable modem and to switch)


• 3 Airport Extremes (connected to the switch)

The AirPort Extreme exposes a WPA2 Enterprise network and a WPA2 Personal guest network. The main network's Radius configuration appears to be correct.

When I'm only using the AirPort extreme everything works well, except there are some parts of the building where the WiFi coverage is spotty at best. As soon as I extend my network with one of the Airport Expresses no one is able to connect wirelessly.

Is there any issue with my configuration that will prevent it from working? Or something small that I may be overlooking?