That happened to me, and I was only just 5 weeks along (I thought I was 6 weeks but I ovulated late). A week later they saw the fetal pole and a little embryo, and I even heard the heartbeat. It might just be too early.
I didnt get a positive test until i think the 7th and it was very faint! I have taken quite a few tests since and the line is as dark as it will get now! I believe i was having implantation early last week that lasted a few days but has since stopped.
It's very possible you had late ovulation or implantation. When did you get your positive test?