A good percentage of Voicebuds we've received back for refunds or defective exchanges test 5-20 db below the normal range because the tube has a tiny bit of earwax or once had a sweat bead suppressing the sound. Cleaning the tube should fix this.
You could try switching to the closed ear domes and raising the volume on the Voicebud.
We also have a feeling that some customers are lowering the volume instead of raising it. Many of the Voicebuds we have tested are on the lowest volume settings. On the Voicebud the button with the "nub" actually lowers the volume not raises it.