Don't know how true it is, but the midwives at my hospital are always reluctant to use the doppler before about 16 weeks because they say it can be extrememly difficult to find the HB.

Good Luck, and I have my fingers crossed for you.
The midwife said that to me too. She was only willing to at 16 weeks because we'd just had an ultrasound and had seen the heartbeat, so we wouldn't stress if she couldn't find it.