Recording using built-in MacBook Pro mic(s)

This is essentially a copy of a question someone else asked but didn't get an answer to . . . I'm recording acoustic guitar with my MacBook Pro's built-in mic on GarageBand, and the input volume drops off a few seconds into the track. It's not related to clipping or the way I'm playing. The MacBook microphone seems to have a built-in noise gate or some other setting that messes up the recorded volume. Any ideas about how to troubleshoot this? I have turned off compression, feedback protection, gates, and everything else I can think of in GarageBand. Recording this way to Voice Memos seems to work fine, so it *could* be something in GarageBand I'm missing, but it's just weird. It does sound muted or gated or compressed somehow.