There are 2 things that may be the issue.
First things first. When it comes to buttons on ribbon cables on handheld systems, most ribbon cables use a common ground which would eliminate an unconnected ground. Since the power button does not work but the volume buttons work that eliminates a bad ground.
My guess would be that the pin on the connector on the main board may be slightly corroded or the original ribbon cable is slightly torn. I would double check under a magnifying glass or a microscope (if you have one) to see if you can see any broken traces or tears in the ribbon cable as that would cause your issue.
The second possibility (less likely) is that the original cable is serially paired to the handheld and if that is the case no other cables would work with it.