I say you find a friend and ask for help. Tell them you're having trouble hearing, and can they use their stethoscope, put it in the right place so they can hear, then give you the earpiece? I do this pretty frequently with weird heart sounds. I know they're there, but I can't hear them, so when someone else can I ask if they can find it then let me listen while they hold the bell in the same place. Takes away the variable of placement, plus they can tell you exactly how it sounds/what you're listening for on that particular patient.
Also, have you listened on yourself? One day I did that and it was super helpful. I could experiment with different bell placements and breathing patterns and see how they sound without the pressure of being in a patient's room.
ALSO, sorry it's small and silly, but make sure your bell is turned the correct way.