Consider these solutions only if nothing happens when you are trying to use Flashnote Derby in Listen mode (i.e. play-to-answer).
First, insure that Listen mode is selected as the answering method.
If Flashnote Derby still fails to hear you playing notes to answer, make sure the app has permission to access the microphone (system settings).
You must hold down the note long enough for Flashnote Derby to analyze and determine which pitch you are playing. Try holding the notes longer and/or playing them a bit louder, especially if you have lowered the sensitivity setting to mask background sounds.
In particularly noisy environments, Flashnote Derby may be unable to determine the pitch you are playing regardless of the sensitivity setting or how long you hold the note.