Some XBee module pairs won't talk

I have a few XBee Pro S2 boards that I’m setting up in XCTU. Let’s say I have boards A,B,C,D connected through USB dongles.

I load Router API into all and write default settings.
Now A and B will find each other, but won’t find C and D.
C and D will find each other, but won’t find A and B.

What on earth? I’ve tried reflashing, switching around the ports and dongles…

Bump – any ideas?