I think you tried already a lot of things ...
Waht i would do .... is to find a non hashing card and place in another computer...
Start the miner program let in run for a while , and check which gpu fans are not spinning , or just check the temp with yuor hand or any infrared thermometer . Remove that card ...
I just built 2 rig each rig have 8 of those cards what you have . I had a problem with one of the card. It was hashing way under 20MH/s on ETH out from box.
Took the card of from the bulit , placed in my "test bench" pc , found out this is the card what have Samsung and Hynix memory , it was the hardest for me to find a right timing , overclocking/undervolting settings for the gpu ...
To me its looks like you have a power issue with your rig , i might wrong ..
What memory your problematic card have ?
Samsung / Hynix / Micron / or mixed memory on the card ?
not sure what mem it has
Its always the same card what hashing with 0MH/s ?
( try switch around pcie slots , and see if the same card is the non hashing card )not always the same gpu, if I close the claymore zcash miner and open it back up usually its a different gpu that's not wanting to hash
Do you have a monitor or dummy plug connected to that non hashing GPU ?
nope, its connected via riser to one of the x1 slots
Did you get the cards from a trustable retailer ?
(Soemone might aleady screwed up the bios and sent that card back , reflahsed the card with the wrong bios before he sent it back )newegg
Did you ever tried the card in another computer ?
yes, actually I had a different mobo that the gpus were on before this one and same problem
Did you tried your rig with another psu ?
1500 watts PSU should be ok with 6 RX 580 but you are pretty much pulling the max from that psu , if you dont undervolt your gpus and your mem's .
yes tried with 1200w atx psu, 1400w server psu and with it is sitting with now is the 1500w server psu
Did you tried to place the card on antother rail on the psu ?
its a server psu
Did you set the virtual memory to 16GB minimum ?
yup its actually at a little over 17gb
Do you know what bios versions yor cards have ?
( i got my cards with a few different bios version numbers : 113-BE366EU-Z48 , 113-1E366CU-S5S , 113-1E366CU-S52 adn a few more )not without checking, I assume id have to load up a program to find that out or is it on a sticker on the gpu?
how many watts total would you say one of the rx 580 8gb SE's from sapphire tech use? in turbo and in silent, seems like its close to around 250w per gpu with NO undervolt
btw I too see this as theres not enouh juice for all the gpus because there not undervolted, having someone else say the same thing does confirm this might be the culprit, I do have an extra psu atm, so I think this next step is to separate couple of the gpus from one psu and put it on a second one,
and see where that gets me