Maybe you are not accustomed to how it feels, especially if this is your first pregnancy? :) I personally took quite some time to realise my baby was kicking then, I just thought it was my stomach growling/churning.
First pregnancy might take longer since you don't know what it feels like. You can try lying down on your back and relax, don't do anything just pay attention to your body, then you may feel some sensation.
Haha i am also 21wks and sometimes i dunno if is baby's movement too.. 😂
it is normal. most impt is during scan, bb is healthy n good