Can you show the code, So that it will be easy to figure the problem.
But, While creating worksheet have you specified, 
useStyles: true
var workbook = new Excel.stream.xlsx.WorkbookWriter({
    filename: filename.xlsx,
    useStyles: true
});
See below code, If it could be of any help - 
var worksheet = workbook.addWorksheet(date);
    worksheet.columns = [{
            header: 'Col-1',
            key: 'col_1',
            width: 5
        },
       {
            header: 'Col-2',
            key: 'col_2',
            width: 5
        },
    ];
    for (let i in data) {
        worksheet.addRow(data[i]);
    }
    // set style for each cell
    worksheet.eachRow(function (Row, rowNum) {
        Row.eachCell(function (Cell, cellNum) {
            if (rowNum == 1) {
                Cell.alignment = {
                    vertical: 'center',
                    horizontal: 'center'
                }
            }else{
                Cell.alignment = {
                    vertical: 'middle',
                    horizontal: 'middle'
                }
            }
        })
    })
    workbook.commit()
        .then(function () {
            console.log('Excel export complete!');
            process.exit()
        });