No heartbeat with home Doppler

Hello ladies, I am 12 weeks pregnant and ordered a solo line b Doppler. I have tried finding the heart beat for 3 days with no luck. I'm getting very nervous since it seems every one else found theirs right away. Now I am over thinking everything because my symptoms have subsided and my stomach isn't hard anymore, I'm worried something is wrong. I have an appointment tomorrow, but wondering if any of you have had any issues with the Doppler? Thanks and good luck!!