Buzzer for low battery

I search to activate buzzer when low battery arrive.
I calibrate sensor voltage and it’s good.
If I fly during 10-15mn, battery arrive at 20-21v but no alert buzzer, and of course it’s a crash.
If I test with a 3s battery to see if it’s under 21v, buzzer works.
So I don’t understand why, with my 6S, the buzzer doesn’t alert me during a fly.

I just want to be alerted, not to land.
Is the an issue for this?