Can’t find heart beat.

Vy

So I found babies heart beat at 8 weeks with my Doppler and haven’t been able to find it since. I’m so nervous I wanna call my obgyn I don’t go back in till March 1st I’ve had a miscarriage before but it was 6 weeks. Why wouldn’t I be able to hear the baby now I never had issues with my other 3 pregnancies