Seems like I read somewhere that there is a certain condition (placenta previa, maybe?) that makes it difficult to feel the baby moving.
Personally, I can’t imagine. I always knew when I was pregnant, just because I felt WEIRD. Five pregnancies, and four were detected between 4-6 weeks. One of them was my “WTF, negative test, was actually pregnant and didn’t realize it until I was nearly 4 months along” baby. When I felt those first fluttery squiggles, I knew right away that there’d been a mistake, I’d been lied to by the pg test.