Doctors know more about where to look and have a better Doppler. Just make sure to try everywhere especially lower by your pelvic bone and be sure to push the Doppler down, the heartbeat will sound like horses running
Look up YouTube videos of how to use an at home Doppler for better results. Making sure you are completely flat and also using the Doppler at an angle while pressing pretty hard helps. The tips I found on some videos helped so much!
It's tricky.. depending on the strength of your doppler you'll have to push in further into your stomach to hear anything.. plus some don't recommend using it because it can cause stress if you can't find the heartbeat.. there are other noises that can sound like the heartbeat too so you have to listen carefully.