The chance of you not finding the heartbeat this far along, is very VERY slim. You get to learn where bub lies and you can normally follow his/her movements.
Heaps of women don't find the heartbeat even this far along, and I think this is what causes so much stress. There is an expectation that it would be super easy to find the heartbeat and it isn't always the case. Again, it depends on what kind of doppler it is and how powerful / effective it is in picking up the bubs heartbeat.