I remember having an appointment at 16 weeks, and they couldln't find a HB either. After about half an hour of trying, I was really beginning to freak out!
They got the portable ultrasound though, to have a squizz, and bubby was alive and kicking - was just the doppler couldn't find the perfect spot.

Doppler definately aren't very good before 20 weeks, IMO.
Please don't get yourself too worried, I'm sure everything is going awesome in that uterus!