Hi there, I had this. In my case it was because my placenta is posterior and was low lying. Obviously when my little one started kicking it felt very muted. At your 20 week scan ask them where your placenta is. But right now don't worry. Keep calm. Hopefully youl feel baby alot more soon, I'm now nearly 29 weeks and I can honestly say that I wasn't feeling hard kicks or movements until I was around around 23 weeks.
Hope this helps.