Update

I am really bugged by this rig. I found out that the problem is not exactly plugging the second gpu onwards, it is actually with 7 of 8 gpus that came with the rig. I plugged other two gpus that I had and they worked fine along the only gpu of the 8 that worked.

What is bugging me is that when I got the rig, there were only one gpu plugged directly on the mobo and there were other 7 gpus plugged in risers. And these 7 gpus are the ones that are not working.

I'm still trying to understand what is happening, trying different arrangements. But I'm guessing there is a good chance that I'll return the rig to my friend...