It's common for OB's that don't do an internal ultrasound in the first trimester to wait to attempt a heart beat till 12 weeks with the Doppler. It doesn't mean anything is wrong, but they like to wait vs give a mom a scare if they can't get a strong one before 12 weeks.
My OB always warned me at my early apt it was perfectly normal to not hear a heartbeat with a Doppler until 12ish weeks so not to worry if it sounded weak or they couldn't find it.