I didn't feel anything until 21-22 weeks. Not sure where my placenta was. My digestive system is, how shall I put this, more active than most - I can sometimes feel activity with my hand (now I'm not pregnant), so it's possible that was masking DD's movements. In short, you're not the only one!
My placenta was at the front with my daughter. It was totally different than when I had my other children, I didn't feel any proper kicks and no kicks or knees and elbows poking out as she got bigger. As she got bigger I felt movement and she used to hiccup a lot too which I felt. I missed the kicks though!
With DD2 the placenta was at the back and I felt her moving at 16 weeks.... but DD1s placenta was at the front and I didn't feel any definite movements until easily 22-23 weeks. And was back in for monitoring loads!! Anterior placentas are an arse ache.
No kicks, just felt like a wave of movement in my belly. I went in to hospital quite a few times to be monitored because some days I felt nothing. All was fine it was just the placenta cushioning the movement, I'm tall too so think she had plenty of room. If your ever not sure just go to the maternity unit and they will be happy to put you on a monitor.
I have an anterior placenta and only started feeling flutters around 20weeks but up to 24weeks is normal. Now I'm at 22 I'm getting the odd definite kick which is lovely. If the baby is moving on the scans I wouldn't worry as the placenta will muffle any movements. I regularly lay down in a quiet dark room and concentrate on my belly otherwise I might not feel anything!
I've got an anterior placenta this time and I'm definitely feeling less movement than with my first. I'm 20w now and feeling faint movements but only really for the last week or so. With DD1 I felt strong kicks from about 17w (posterior placenta).
They say you should start feeling regular movements by about 24w, so you've got a while to go yet. Lots of people don't feel anything at 18w, especially with the first, so try not to worry. If you're at all concerned, go in for monitoring. I'll definitely be doing that this pregnancy if the anterior placenta stops me from feeling regular movements, I don't care how neurotic I look!