I had a new s5 do the same thing. Only 1 board would hash. Bitmain gave me a place to send it to and have it replaced. I was the original owner though and they asked for the order number to verify. I also had two units with bad control boards. Not the best luck from the last batch.
They have a good support team, assuming you bought it from them and it's still in warranty is best option.
They will still help out of warranty but parts will cost.