At 6 weeks it's not uncommon to see only a gestational sac or yolk sac. A repeate of the ultrasound in a week or two should reveal a fetal pole and heartbeat. Becuase it's near impossible to pinpoint the exact day of implantation dates can be off by a few days and being this early in a pregnancy a few days can mean a world of difference in what you see.
i went in and they said according to my last menstral cycle i was 7 wks... then i had an ultra sound and it showed me being only 5 wks and no sign on an emryo so i went back 10 days later and the ultra sound showed me being 6 wks 3 days and there was an embryo as well as my babys heartbeat! so... im guessing you are just right at 6 wks if you are even that far bc my dr said you wont see a heartbeat until atleast 6 wks 3 days and possibly not even that soon!! you might not even be able to see an embryo yet!!
Depending on the quality of the machines and if you are earlier than you think, it's very possible. Some people don't see a fetal pole or heartbeat until 7,8, or 9 weeks. I didn't see any of mine until I was 7 weeks and a few days along. Have they scheduled another appointment or done blood work to see if your levels are rising? Another ultrasound would show if there's fetal development and if the pregnancy is viable or not.