Sonoline B Tips, Anyone??

So, I'm currently 13 weeks + 5 days with my miracle baby after a miscarriage. I bought a Sonoline B fetal Doppler second-hand from someone today & I was hoping to pick up my baby's heartbeat. I bought some regular lube at CVS because they didn't have ultrasound gel. I could hear my own heartbeat, but no luck hearing the baby's. The doctor always has such an easy time finding it. 😔 
Tips anyone??? Help me find my baby's heartbeat!