Looks to me like you may have a dual-rail PSU and you are not splitting the cards along rail boundaries. Means that one side of the PSU is providing all of the current and the other is sitting almost idle.
Rail 1 - Card 1 + Card 2
Rail 2 - Card 3 + Controller + fans.
Other possibility is that this is an older PSU - PCIe power connectors (especially the cheap ones) are not rated for many insert cycles.
Or, as said, dust. If the connectors have a layer of dust on before they are plugged in, it's not copper -> tin -> Copper, it's copper -> tin -> random non-conductive stuff -> tin -> copper. Not good.
Good luck. If you want, you can forgo this pain and solder the PSU directly to the board - the tracks are quite thick though so use your big tip.
Rail 1 - Card 1 + Card 2
Rail 2 - Card 3 + Controller + fans.
Other possibility is that this is an older PSU - PCIe power connectors (especially the cheap ones) are not rated for many insert cycles.
Or, as said, dust. If the connectors have a layer of dust on before they are plugged in, it's not copper -> tin -> Copper, it's copper -> tin -> random non-conductive stuff -> tin -> copper. Not good.
Good luck. If you want, you can forgo this pain and solder the PSU directly to the board - the tracks are quite thick though so use your big tip.