I have an angelsounds fetal doppler- which is not any kind of professional one, but it seems to work really well. I've had it for a few of my pregnancies now, and it always starts to pick up the hb around 10/11 weeks. I did have 2 days this week where I couldn't find the hb and got really worried, but then I found it later that night.
There is definitely tons of room for this kid to move around and it seems to like to hide. I also had a scare during my last pregnancy around 14.5 weeks when I couldn't find the hb on my doppler. I called my mw and she said to come in to do a check. SHE couldn't find the hb either! She sent me for an emergency ultrasound which showed a perfectly squirmy baby. But I did have an anterior placenta with him and that can make it harder to hear the hb depending on babe's position.
I wouldn't worry- 12 weeks is still early even for a doppler, and lots of times drs. can't even find it at this point.
Married to DH, Mama to dd1 4/06, dd2 8/07, dd3 11/12, ds 9/14, and #5 due April 2016!