Why is my volume down button not working?

Can you give us more details on what device? Sometimes it can be hardware fail sometimes it can be issue on a board or trace. If you could provide us with more details we would be able to help. Also, does your button click when you press it?

Volume atas berfungsi dan volume bawah tidak

If your volume down button isn't working, the issue could be due to either software or hardware problems. Well Look closely to button and check for any sort damage and try to Restart your device and also check for the software update sometime an outdated software version can be the reason of this kind of issue so check them all.