I am right there with ya (check out my post about this just a few days ago on the Pregnancy Forum). I definately felt the boys a LOT by now, and this one I am lucky if it is once a day.
I just had my anatomy scan (u/s done between 18 and 20 weeks) and read a few days later that if your placenta is anterior, you will not feel movement as much. So I called my ob to find out if it was in my chart and sure enough, the nurse read that my placenta is anterior. I feel a little better. You might want to find out if it would make you feel better too, but the responses on my post on the other forum will help, too.
One woman PM'd me that she didn't feel anything until 23 weeks.