As you know, I LOVE my doppler. Since I'm 12w also, it is a bit difficult to find the HB at this point, and there have been days that I didn't find it. But I know it is normal in this stage, and it doesn't cause me any anxiety. Last year I got it when I was 22w, and I was able to find the HB really fast every time. We get kind of used with it, and it becomes easy. In my opinion, it you are in a stage when finding the HB should be easy, and you try a few times and don't hear it, plus the baby doesn't move, you should run to the doctor!
I have one called Bistos, I chose this one because it's the same kind we use in the hospital here, and it is very good. Well spent 100$!