尝试打印错误消息:

switch ([exportSession status]) {
        case AVAssetExportSessionStatusFailed:
            NSLog(@"Export failed: %@", [[exportSession error] localizedDescription]);
            break;
        case AVAssetExportSessionStatusCancelled:
            NSLog(@"Export canceled");
            break;
            case AVAssetExportSessionStatusCompleted:
            NSLog(@"Successfully");
            break;
        default:
            break;
        }

    }];
}