Swap the PSU's around, and see if the "problem" switches to the other borads - in which case replace the PSU

If that's not it, take it apart _carefully_ and check the cabling hasn't come out for the uart/usb/comms the board from the asics

And if that's not it, you have some boards which have failed
 - they do wear out - about 1/3rd of mine (all underclocked for efficiency/power reasons) died within a year