Tell us a bit more about the mic, the room, the size of the harp. (I assume it's the harp with strings, not the harmonica!!).
It's often a question of mic placement, but the type of mic is important with a delicate acoustic instrument. You might need to run the mic through a preamp.
I've had the same issues with classical guitar.
A mic doesn't "know" if a harp is playing or if you are talking. It picks up sound waves. This has nothing to do with GarageBand. If your setup is working and the mic delivers the right level then the problem might be your placement of the mic and direction. Is it directional or omnidirectional?
You might just have it pointed backwards, recording the room but not the harp signal. Check the mic documentation
You can read up about the best placements for typical instruments but one simple rule for recording acoustic instruments in a room or hall. Walk around and listen to the instrument/ensemble to find out where it sounds the best and start with your mic position there.Hope that helps
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A condenser mic needs power to operate either from a battery or what is called a phantom power source. Your mic has electret in it's name, that I believe means it can create it's own power or has it's own power.
But seeing you can record your voice fine the power is not the problem. My guess is your mic is not sensitive enough. You may need to try moving the mic closer or find a sweet spot in the room as suggested above. Or try the built in mic.
I've never tried to record a harp (probably not the only one) but I think you will need a mic that is very sensitive with a wide frequency range. Probably best to call one of the big online music retailers and get a suggestion on something in your price range. Sweetwater would be a good place to try.