Some ios device(such as iPhone 6 Plus) has letter box (black box)

Hello,
i make application to support meal-table in company.

I publish this application into iTunes, and i received an report from user.
“Sir, This app has black box in iphone 6+. do you check app?”

so, i tested my app use ios-sim, it have blakc box like below photo.

How i solve this problem?