This is kind of personal, but are you very heavy? If there is a lot of fatty tissue between the doppler and the uterus, it's possible not to hear the heartbeat.
Also, are you sure of your dates? Is it at all possible that you're less than 11 weeks pregnant?
Sometimes if the uterus is tipped backward instead of forward, that could be a problem, too. A percentage of women have their insides arranged that way. In that case the weight of the baby eventually pulls it forward.