12 week appointment Doppler couldn’t find heartbeat [bc]

Hi, wondering if anyone has experienced this. I had an ultrasound at 11 weeks and everything looked normal and I saw the flickering heartbeat. Today at my 12 week appointment the midwife said everything looked normal on the ultrasound and all my blood work looked good. She tried to find the heartbeat for about 6 minutes with the Doppler and wasn’t able to hear anything. She said there was a 50/50 chance this would happen but now I’m worried! I go back next week for them to try again. Has anyone experienced this? I’m 12 weeks 1 day