The same thing happened to me at my 16 week appt. (which was actually done at 17 weeks). She couldn't find the heartbeat, and finally resorted to ultrasound, where she was able to locate it more quickly.
She said that at this stage, sometimes the babies move around too fast to catch them with the doppler. Once she said that, I had to laugh, because I had a mental image of baby racing around my uterus trying to escape the doppler :)