I haven’t dusted off my Platform Cable II since I created the Mojo, but I just hooked everything up to an Au+ and Au and it detected the FPGAs right off the bat.
Mine is the red version and the pinout looks slightly different (mirrored) but I can’t imagine it matters.
I did power both of mine over the Au’s USB port instead of via the Br board. My only thought is you might be creating some grounding issues between the platform cable and the bench power supply. The platform cable seems connect its ground to the USB ground.
I’m not sure what the difference in the cable is, but I connected both GND and PGND to the Au’s ground. Connecting either of them made the FPGA show up though so I don’t think this is necessarily the problem.