The fact that you see peaks at 2f, 4f, 6f and 8f strongly implies that either your input data is actually an octave above what you think it is, or that you're misinterpreting the frequency scale of your results. If you were just missing the fundamental frequency, you'd see 3f, 5f and 7f as well.
Suggestions:
- Plot your input data before you FT it. You should be able to eyeball the frequency of the dominant term.
- Listen to the note produced by garageband. Is it above or below middle C?
- Check that you understand where the values on the frequency scale on your plot came from.
