Mssleepyhead, this is your first, isn't it? With my first, I didn't feel a thing until after 20 weeks. Not a thing. Some people do, some don't. Just because you're not feeling it, doesn't mean it's not wiggling around like mad.
Second time around, I have just started feeling a few definite movements at 18 weeks. Two weeks ago I had the faintest flutters, and I know that the first time around I would not have recognised them as the baby. It's only knowing what they are that makes me aware of them.
I have never used a doppler, but I do know that from week to week things are changing a lot inside the womb. Surely that must mean you're going to be hearing different sounds at different times? If it starts worrying you more than comforting you, perhaps put the doppler aside for a while?