When should I be worried?

I’m 17 weeks pregnant with my fourth child and still don’t feel any movement. With my first I felt baby at 22 weeks, my second I felt at 17 weeks. With them I never knew where placenta was, atleast nobody told my if they where in the front. With my third the placenta was in the front, and I felt baby move at 15 weeks. This time they haven’t told me where placenta is, but I think they would’ve told me if it’s in the front.When should I be worried about not feeling movements yet?