Hey, thanks for all your kind replies  

The problem turned ou to be the SATA to MOLEX cables that were a bit too bented, and therefore couldn't power the risers !
I put them in a better position and the cards were finally detected.

Now, I flashed the BIOS of the cards (with Polaris and ATIflash), but the fans don't spin anymore (only a bit when I turn on the MB, but when W10 boots, they go off and get really warm). Prior the flash they were always spinning !
Another fun story : once I flashed them, the memory type changes on GPUz (from Hynix to Micron, and then from Micron to Samsung...) I didn't take a random BIOS on the net, I just clicked "One click timing patch" on Polaris, saved the new .rom file, put it in the ATIflash folder, runned CMD and flashed one card at a time...  However, on MB the memory type is the right one. So I reinstalled GPUz, and the memory went back to normal (except the fans, that still do not spin)

Should I wait more, and hope that the fans finally decide to spin? I'm afraid that they never will, and that the card will destroy itself  

This rig is really driving me crazy  
Help and advice would be very appreciated