Saw heartbeat but can’t hear it?

I’m 7+6 today baby measuring 8 weeks, I saw heartbeat today on scan looks great, I’ve tried to find it with my sonoline B Doppler and I can’t find it at all? I’ve watched YouTube tutorials and I can’t find baby’s heartbeat at all with my own Doppler. How come I can see it on the scan but not hear it at home?? Thanks