I have made several of these cables- IIc to Mac, IIc to PC.
What you need is essentially a IIc to Imagewriter I cable. The Imagewriter
I used a DB-25 connector. Your pinout matches mine, except I don't have a
connection to pin 5 of the IIc. I doubt if that is the problem, though.
There is a major pain, which you've discovered. Some of those connectors are
numbered one way, some another. There is also the confusion factor of
whether you are referring to the mating side or the solder side of the
connector.
Since it seems like it should work, I'd suggest reversing the order of
connections, and then if that fixes it, get back to us with a diagram that
includes whether you are referring to the external mating side of the
connector or the side that the wires are soldered to.
-Paul