Hi, there can be different reasons why your having this problem. First if glandular tissue was removed, too much may have been removed under the nipple to cause this depression. Secondly, it could be due to excessive liposuction in the area. Depending on what is causing the problem, your plastic surgeon may offer to due a revision surgery to release the scar tissue that may be causing the problem +/- fat grafting to area to restore the sunken appearance. Or surrounding fatty tissue could be liposuctioned to improve contour. Would be best to let your plastic surgeon examine you to determine what is causing the depression.