VMA309 sensibility

Hi,
I just bought a vma309 sound sensor.
I connected it to my raspberry pi zero and every thing went well except for his sensibility.
After adjusting the capacitor, his sensibility just “goes off” after a few minutes. I have to scream to make the led turns on. I adjust it again, and again, and after a few minutes I’m at the same starting point.
What am I missing?
Thank you for your time.