How do I combining sorting in gridview with checkboxes vb asp.net

Hi!
On my webpage. www.antagningsstatistik.net/default.aspx  I have checkboxes to filter the data displayed in the gridview. I works fine with multiple selections. However, I have also enabled sorting on column headers in the gridview using the smart tag in Visual web developer.

When I make a selection with for instance to selections in the checkboxlist and click the button it looks fine. But when I click on a column header to sort the gridview based on the to selections, only data from linked to  one of the two selected values from the checkboxlist will be displayed.

I would apprecitate if some one could give me a hint or maybe even a solution to what is wrong with my code.



I also attach the files here.

 default.aspx
default.aspx.vb