When we swapped these units in the field they acted as if they had never been connected.
Did you enable full logging on the ePMPs and see anything at all logged? We've had pairs of ePMP PtP radios do this to us, and in most cases I can see log entries at the AP end indicating it's not allowing the connection due to authentication failure. Repeated reboots of each end sometimes leads to link up, usually I've had to disable WPA2 on that AP and the link pops right up.
Ensure that you have the max-range set under Configuration -> Radio on the access point set further than the link distance. I think it is set to 2 Miles by default.