My iMac will not connect to time capsule used as base station in my roaming network

I have a network that is made of up a Time capsule as a base station and 3 airport express's used to extend my wifi coverage. It is setup as a roaming network with each express hardwired to the base time capsule. I have a iMac that will not connect to the base station, it will only connect to one of the remote express's. The iMac is next to the base satation. Other mac notebooks, iPhone or iPads all connect to the base station when they are near it. Only my iMac will not connect to it. Any suggestions as to why this is happening or steps I can take to fix this.