Aiyoh don't worry lah, week 16 memang sometimes tak dapat lagi especially first baby, fetal heart tone biasanya clear around week 18-20 dengan doppler, tapi depends on position baby, placenta, and uterus position jugak. A few of my mum friends pun had to wait till week 18-19 baru nurse boleh dengar dengan jelas, so it's totally normal! Kalau you're worried, you can always ask your doctor to check at your next scan or appointment, they'll confirm everything is fine there.