Doppler

Hey guys! I had a doctors visit this morning and she had a listen to the heart beat on the Doppler but what I’m worrying about is the last time a doctor checked on the Doppler I could really hear the heartbeat perfectly but this time I couldn’t really hear it, I could hear something but not like the normal heartbeat sound! She said everything is fine and on the monitor it’s all normal but what would be the reason for me not to be able to hear is like the first time ?

Sorry I’m a first time mom and worry about everything. I’d love to know your experiences ☺️ xx