Its normal. My son always kicked before 20 weeks but with my 2 girls I hardly felt them, I'm currently 28.3 weeks and I'm pregnant again with my 4th and I could feel her kicking at 18.3 weeks and from on. I think it just depends on the baby.
Your baby is still small. I'm almost 21 weeks and I have very strong kicks. Every one is different. If your concerned talk to your doctor about it.