6 weeks heart flicker, no heart rate

Sierra • Mother to Elijah🧡 Braelynn Rose 💜Kai River💙, Lillyana Rayne💗, Karsen Grey 🩶, & Aydin Storm 🩵

I’m measuring 6 weeks exactly, exactly along with my LMP. They could see the flickering of the heart, but not pick up what the heart rate was. Is this normal?