Heartbeat with doppler

Can 14 weeks till be too early to hear baby's heartbeat with a doppler? At 12 weeks my dr had to do an ultrasound since she couldnt hear the heartbeat with the doppler everything was fine.im 14 weeks and tried to listen to my baby's heartbeat with my doppler and I still cant find it.. it makes me sad because I still haven't got to hear my baby's heartbeat.