Fetal Doppler- sonoline B

After viewing raving reviews about the sonoline b fetal Doppler, I bought one and received it yesterday. I didn't ever hear the heartbeat, after searching for almost an hour! I bought the Doppler for peace of mind and if anything now I'm more worried. My first pregnancy ended in miscarriage at 9 weeks the baby didn't have a heartbeat at ultrasound. This pregnancy I'm 12.5 weeks and at 9.4 weeks the baby's heart rate was 171! Thank god. But that was almost 3 weeks ago and I couldn't hear the heart beat on the Doppler last night. I'm scared to try again. Any one else relate to this? Thanks for listening baby mamas!