Baby Doppler

Anyone find it harder to find baby’s heartbeat as you become further along like 14-16 weeks? I found my baby’s heartbeat immediately when it was 9 weeks but now it’s been taking a minute to find it and even when I do find it, it eventually goes away or I’ll lose it. Is it because baby is actively moving around? I also sometimes find myself hearing both my baby’s heartbeat and my own at the same time. Any input would be greatly appreciated.