A bad HUB / Cables could be an explanation. Check this. Sometimes it could be feedback power over the USB cables as well. On my Ztex boards i have something similar to a clear CMOS Jumper to reset the boards configuration.
Did you try to connect them solo each board, or all together?
Try to conect solo each board, so you can determine, the two are OK or not.
Basically I have spent a few hours swopping and changing cables, I have rotated the boards starting them independently and together, Its the same two boards I cant get to recognize.
I have also used another computer, the first three are all good, The other two, Nothing. not even a sound/bubble to say I have plugged something in.