Did they tell you where your placenta is? Anterior placenta makes it difficult to feel baby.
At your anatomy scan, did your baby move a lot? Or were they kind of a lazy baby? Your placenta might be on the outside instead if the inside as well, which would make it harder for you to feel movements. As long as you feel something, though. :)
Are you a heavier girl? I heard if your a little on the bigger side it's harder to feel the baby move until later in pregnancy. I'm 24 weeks and feel him rolling and kicking all the time. But I am also average for my weight. Try drinking something cold and laying on your left side. Usually that gets them moving. If you are truly worried I would call your doc or go to the er they will give you an ultrasound to make sure everything is ok! Good luck!