I have solved the issue by adding this code into my function.php.
/*************************************************************************
Sorting: source= http://wordpress.org/support/topic/sort-posts-alphabetically-1
***************************************************************************/
function wpsf_orderby( $query ) {
if ( is_admin() || ! $query->is_main_query() )
return;
// do conditional checks here and return on false.?

{
remove_action( ‘pre_get_posts’, __FUNCTION__ );
add_filter( ‘posts_orderby’, function() { return ‘ post_title ASC’; } );
}
}

add_action( ‘pre_get_posts’, ‘wpsf_orderby’ );