Almost always permissions. Your phone has to have given GafferMate permission to use the microphone, and it only asks once.
iPhone: Settings → GafferMate (or Safari, if you're using the website) → Microphone.
Android: Settings → Apps → GafferMate → Permissions → Microphone.
Other things worth ruling out: another app holding the mic (a call, a voice note, a car Bluetooth connection), and a case or a thumb over the microphone hole.
If permission is on and it still won't record, tell us what phone you're on — hello@gaffermate.co.uk.
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