I am a bit puzzled as to the exact application. You say into a soundcard and an also tried it into an amp. Does your audio interface not have a phantom power supply..same applies to the amps mixer. Most do have phantom power already. Why are you using a separate dedicated phantom power source?
Even with the AT8801 you should get a signal especially if another mic works using the same setup. It is possibly a dodgy mic. I would try it out with another phantom power source. I would also try another cable. I know that some Rode mics have their own cables wired up different and are not compatable with other mics.
With what you say it can only be a dodgy mic, dodgy lead, or dodgy power source. Process of elimination to test them all out.