Could be the way they are lying. I felt mine first move at about 15 wks but only every so often. As they get bigger you should feel more movement. MW will say the same. Try not to panic yet, you don't want to stress the baby out. Good luck
Am 18weeks pregnant too... I barely feel movement not like when I was 13-16weeks....ain't worried dou, cos am going to see my doctor nextweek