What exact procedure of maintenance do you do? Did you just use compressed air to clean them up?

How about the temp and fan speed?

If it worked before, I believe this undetectable ASIC chips has poor contact with the heatsink.
If you can disassemble the unit again, I recommend cleaning it up, removing the old thermal paste, and replacing it.

Also, test them one by one because sometimes other hashboard affects the other hashboard to send false signals.