When I scroll the ag-grid table, header does not seems to be fixed on top and just hides. What seems to be the problem?
I have imported all the css required.
It's likely that your grid is overflowing inside of the element that is hosting it, causing the normal browser scrollbar to appear. You are scrolling the entire grid element, not telling ag-grid to scroll. To prove this set a height on the containing element of the grid to something small enough so you can see the bottom of the grid.
see Grid Documentation on height.