It depends on where the placenta is and whether or not you are over weight. If you have an anterior placenta (towards the front) or are overweight, it definitely can take longer to feel things. Also if you are a ftm, you just may not realize that's what your feeling. The initial kicks you feel almost feel like muscle spasms or gas bubbles. If you are concerned, however, never hesitate to call your health care provider.
Some women don't till later on usually it's not seen as a problem till 28 weeks when they want you to start counting kicks