It appears you did have a reasonable outcome from your genioplasty. An 8mm genioplasty advancement is a fairly large advancement(around 10mm typically being the maximum for an osseous genio adv). I am guessing you did not have an isolated small chin, you likely were retrognathic and your entire mandible(lower jaw) was small also. Your teeth may not line up completely appropriately and a better surgery for you may have been a sagittal split adv of your mandible in addition to your sliding genio. I recommend discussing your dissatisfaction with your surgeon and see what he/she can offer to help you. If they cannot help you, seek a consultation with a properly trained Oral surgeon or plastic surgeon, in NYC I think there are even a couple of surgeons that are trained in both Oral surgery and plastic surgery.Good luck