Heartbeat visible at 6 weeks?

I am pregnant with my second baby and am having some issues. I started bleeding and spotting early this week and immediately went to my doctor. I was sent to get an ultrasound and they said the sack and embryo both measured at 6 weeks but no heartbeat was visible. They said it happens sometimes but they are repeating it in a week. Also I am having my hcg levels taken and my first was 12518. My question is has anyone else had an ultrasound at 6 weeks and we're not able to see the heartbeat? I'm trying to hold on to some hope that it was just too early to see.