Can't find heartbeat at 12 weeks

So I have a Doppler at home.. Since 8 weeks I have been able to find the heartbeat every time I tried.. I turned 12 weeks yesterday and can't find it after multiple attempts. I'm ver worried because I miscarried with my last pregnancy. I'm hoping that bc the placenta has been growing that it is now in the way making it difficult to hear. My next appointment is not for another week... I'm not sure how I can wait that long to be sure everything is ok. Has anyone else been able to hear a heart beat and then all of a sudden can't?