DHTMLX grid sort

I have a grid with 2 header rows. When I used to have 1 header row the sorting was working perfectly. Now when I click on the header to sort the row, the rows are not being sorted. Is there anything special that needs to be done to enable sorting when having more than one header row?

There is nothing special, but please beware if row has colspan it used sorting as it defined for leftmost column in colspan.
If problem still occurs for you - please provide a sample of js code which cause incorrect behavior.