ExtJS - CheckColumn in the Grid does not appear in production build

I have a checkcolumn defines in a grid. When I run the appliation locally, checkbox is appearing fine. But when I built it nad upload it on server, I can't see the checkbox appearing. Entire column appears as BLANK.

Code of the checkbox column:

{
                                            xtype: 'checkcolumn',
                                            dataIndex: 'FLAG',
                                            text: 'Set Flag?',
                                            flex: 1,
                                            listeners: {
                                                checkchange: 'onCheckcolumnCheckChange'
                                            }
 }

FLAG is defined as Boolean inthe model:

Ext.define('App.model.FlagModel', {
    extend: 'Ext.data.Model',


    requires: [
        'Ext.data.field.Boolean'
    ],


    fields: [
        {
            name: 'ID'
        },
        {
            name: 'DS'
        },
        {
            type: 'boolean',
            name: 'FLAG
        }
    ]
});

I suspect that some of the CSS file is missing while building the app.

What can be the reason and how to rectify it?