Mummylove-are you sure that yours is a doppler, and not just a fetal heartbeat listener? I've seen a few of those at places like Babies R Us, and the supermarkets. The baby is too tiny to hear with a microphone yet. Just a thought.
The Angelsounds one is great, I've found it every day since 12w in a couple of minutes.
The BabySound B is normally pretty well reviewed. I can only think that perhaps it's the position that you are looking with. Have you tried running it very slowly across the top of your pubic hairline? And are the batteries still strong? You have to look in all directions as you move it across, and it takes a bit of pressure too-I can't just rest it on my belly. I do it like the doc did with my first pregnancy.
Good luck either way hun, I hope you get one working.