Maybe your earlier than you thort?
Sometimes it just takes longer to be able to see it, I've heard of a few people it taking longer, or maybe you're not as far along as you thought. As much as the wait and see game *****, it'll be worth it if you see a healthy baby growing next U/S, I'll be thinking of you, and hoping everything works out for you!!
I'm wondering if your not as far along as you may think. With my last pregnancy my Dr said I may or may not see the heartbeat at 6.3 weeks because it was still very early. Everything was so small then too. As hard as it is to wait for the next ultrasound it's all you can really do at this point. I know it's an awful feeling to have to sit and wonder what's going on but it gives everything a chance to grow and hopefully be seen.