Heartbeat???

Ok ladies, I went to my 2nd Obgyn appt this morning. I am 9w4d and baby measuring at 10w2d. Doc did heart doppler but couldn't the heartbeat. He did a sonogram (non vaginal) and we saw the flickering of the heartbeat but couldn't hear on doppler. I am a little perplexed as to why? Any ideas or experience in this? Doc said everything looked great and bloodwork came back all fine and said I am right on track and so it baby.