Could be that they are both fighting for the same ip address. They're both typically in the 192.168.xxx.xxx range. But if they are both wanting to be called 192.168.0.1 for example, they will never be on speaking terms. Their native addresses should be on the id label of each bit of kit.
Alternatively they could be wanting to be on a different subnet. I recently had a repeater which wanted to be 192.168.2.1. Had to reset it to the '0.xxx range, then it worked perfectly.
hope this makes sense?