Sometimes the heartbeat can't be heard until about 10 wks. I know a girl who was told she had a miscarriage due to no heartbeat detected at 8 wks. She's now 21 wks and her baby is perfectly healthy. Baby is probably still too small.
There's a chance that the baby may still be to small or the baby can be hiding so they can't get it. Keep your head up an be strong
I was told the same thing.. im still waiting for the ultrasound