I just had butt implants put in about 2 weeks ago. ?

The implants look nice and round on top right under my back but, the middle of my butt is flat and the ending of my butt isn't too rounded up either. My plastic surgeon said the implant will gravitate down alittle so I would fill the flat part in. I see pics of other implants and they look nice and round all over. I would appreciate someone that has had this problem or another plastic surgeon to give me advice on what to do and what to believe.