There are two main reasons which could cause this issue.
1. Volume Mixer settings on PC
Window's Volume Mixer settings allows you to adjust the volume on a per app basis. Please ensure that your PC's Volume Mixer settings for the app playing audio has not been set too low.
(To access the Volume Mixer settings Right-click the speaker icon on the Taskbar -> Select "Open Volume Mixer" or "Volume Mixer")
2. Receiving Bluetooth device not accepting PC commands.
The Leaf is a USB audio transmitter for streaming digital audio (USB audio and Bluetooth audio), as a result the Leaf cannot be used to directly adjust the volume. When the Device volume is adjusted on the PC, the Leaf will PASS this command to the receiving Bluetooth device. If the receiving device doesn't accept this command from the PC for any reason, then this can also cause volume control issues.
Device volume on PC
In this case, please try adjusting the volume from the playing app. e.g. YouTube on Chrome as shown below.
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