I thought I was just doing something wrong in using it until I consulted my midwife friend and she said I should try listening to my own heartbeat first. I couldn't even hear that, and I couldn't hear anything when I tapped the bell, either. Hmmm. I took it apart to see what might be causing it, because it's all supposed to be hollow to allow sound to travel. I blew air into the bell and there was a blockage. I looked more closely and it was full of yucky, thick black stuff, and I'm still not sure what it was. I bent a dental tool with pliers and scraped it out after soaking the bell in scalding hot water from the stove (I did this in my bathroom sink, not in a pot I would be using for food!) and I was able to get enough out that I could blow air through the bell and through the tubing. That was SO GROSS.
All that said, yes, it worked after I went through the nasty process of cleaning it. I found my baby's heartbeat easily at 25 weeks. I would have returned it instead of attempting to clean it but the window for that had passed.