Another thing is chest shape. My husband has a rather barrel shaped chest due to his pectus excavatum (his breast bone caves in but the rib cage bows out). It's very hard for me to listen to his heart or his lungs. I can hear them but I have to find the right spots. My heart and lungs are easy to hear with the stethoscope. And yes, some stethoscopes are much better quality than others.
Difficult for YOU to hear with a stethoscope or difficult for a DOCTOR to hear with a stethoscope?
If it is YOU who's having the trouble, there are millions of reasons ranging from inability to distinguish low decibel sounds, to earwax. Could be that you have an old or poorly maintained stethoscope. and on an on.
If it is the DOCTOR who is having the difficulty and it's not just the doctor but the nurse and others as well, then it could have something to do with you...otherwise, it can go right back to trouble with the listener.
If it has anything to do with your system, it could have to do with weight. You have not indicated your weight, but if you were heavy, the body tissue seperating the heart from the stethoscope can distort the sounds. If you are of average size, then it could have to do with other sounds in your body....are you asthmatic, have a cold or digestive issues that could mask the beating of your heart with other internal "noise?" If the answer to all of that is no, and the listener has good hearing and a good scope, it could be that your heart is truly beating with decreased force. This is a concern and would require follow-up to ensure there was enough pressure created by the pump to sustain blood volume and availability to the extremities. If your doctor did not recommend any follow-up, or did not check your blood pressure several times after this occurred, then you have nothing to worry about and can just pass it off as one of those weird things that happens in life.
Take care.