In my pregnancy we were not able to see the heart beat until 8 or nine weeks. They actually though I miscarried because I was originally pregnant with triplets. They were getting ready to do a DNC on me and dr. did another blood test, and hcg levels were rising. We had an ultra sound at another hospital and there was my little peanut heart beating strong. Remeber your body is different, no two are a like. Scar tissue, to much belly fat, or excess skin can make it hard to see. You could have also got a bad tech.
Just hold on, and don't start think negative yet. Everything could be just fine, and you are stressing your little peanut! Good luck and you are in my prays.