Heart sounds naturally vary from patient to patient. Obese patients have attenuated heart sounds, as do some older patients. Chest hair and some clothing, such as knitted or thick garments, can reduce the coupling between the diaphragm and the chest. Finally, sounds are dependent on the location on the body.
There is a technique to using One well. Start by applying a light pressure on the diaphragm. As you increase pressure, One becomes more sensitive and volume increases. There is a "sweet spot" where diaphragm sensitivity is at a maximum. Then if you push even harder, the diaphragm will touch the sensing plate behind it and its vibration will decrease. This is similar to conventional stethoscopes, where pressure of the chest piece had an effect on sensitivity. We've kept that tactile feel on One, along with other abilities to control the sound. So vary the pressure of the diaphragm against the patient and learn to find that "sweet spot."
Another obvious solution is to adjust the volume level and filter setting. Try increasing the volume and setting the filter appropriately to capture low frequencies for heart sounds.
Finally, there's the question of headphone seal and headphone quality - whether you are using good headphones. Check out our advice about which headphones to use.