It can be because of adenoid enlargement at this age, better to get an X-ray of the adenoids done. This will show if at all any block is there. Also, you can try instilling saline nasal drops 2drops 3 times/day. If possible consult another ENT specialist.
Has she ever had a ct scan? My daughter suffered sinus and ear troubles. The doctors said it was allergys. After I pushed for a ct scan it was discovered that she had a safety pin in her nasal cavity. So, a ct scan would not hurt.