Sometimes I have a hard time hearing out of my ADC stethoscope. Why?

There are a number of reasons that could happen. One is called the binaural effect. In the binaural effect sound presented to two ears is judged to be louder than the same sound at the same loudness presented to only one. To illustrate this phenomenon, try this experiment. Listen to music though earbuds and remove one from the ear without touching the volume. The perceived volume will drop dramatically.

What does that have to do with your stethoscope? Well if the headset isn't properly aligned, the spring tension is too loose, or the eartips don't make a good seal, the sound transmitted to one ear may be impeded. You will perceive this as a dramatic drop in volume, or, at the very least dramatically reduced sound on the “disconnected” side. This can also happen if you hold your head in a certain position during the auscultatory exam.

Take these steps to ensure a proper aural fit and maximum acoustic performance.

Headset Orientation: Make sure the eartips point forward TOWARDS the bridge of your nose. This is the number one cause of diminished performance in a stethoscope.
Eartube Alignment: Make sure each eartip sits snugly, firmly, but comfortably in the ears. Adjust as needed.
Eartip Fit: Large and small Adsoft are provided. (small are on the scope). Some models include a second pair of small as well. Pick the size that provides a comfortable fit and good acoustic seal.
Spring Tension: Adjust the tension for a snug fit but make sure its comfortable. Adjust as needed

As to head position, most professionals find that for best headset alignment and acoustic seal, the head should be tilted down slightly.