At my last OB appointment 10 days ago, my doctor couldn't find the heartbeat with the doppler either, even though I was 1 day shy of 14 weeks! Luckily, he has an US in the office and there it was -- my little baby. Funny, he was able to distinguish the sex (boy, although not 100% sure at this point!), but we couldn't hear the HB.
Try not to worry, but seek out some reassurance if necessary! Good Luck with your little one!