Your placenta could be in the front which could make you not feel the kicks as early or as strong...it's nothing to be worried about if that's the case, its just because the placenta is cushioning the baby's movement, but soon he or she will be big enough for that to not matter ;-)
I was 22 weeks with my 2nd and 19+4 with my 3rd. No worries! I freaked out with my 2nd and was so worried.