My colleague who had her little girl in April didn't feel any movement until nearly 30 weeks, as her placenta was all at the front.
Apparently, according to one of the books I have, you don't feel the movement in the womb itself, you feel it when that movement presses on the outside of your abdomen, os if your placenta is in the way, it's like having a big pillow between the baby and the wall of your abdomen where you can feel things.
So try not to worry, it's just well protected in there!
D