Bone conduction requires firm contact with your head in order to work correctly. If the transducers do not sit firmly against your head, particularly the bones around your ear, then it is possible that the audio may sound off or be too quiet.
To tell if the bone conduction is working, plug your ears while listening to something. You should be able to hear the audio clearly when your ears are plugged. This is because the audio is being conducted through your bone into your inner ear.
If the glasses fit, try changing your equalizer settings on your phone. Keep in mind that bone conduction is prone to leaking due to its use of vibrations, particularly in quiet environments.