sonoline b help

So I ordered the sonoline b Doppler.  I'm 12w3d and haven't been able to find the heart rate yet.  I'm not worried it's not there, just disappointed because I want to listen to our baby! I've watched numerous videos and have tried going slow and low, but no luck.  I know they don't try in the doctors office until later for this reason...but grrr! Any tips? Anyone else have trouble finding it til later on? I'm also overweight which I know can play into making it more difficult.