Crystal Reports :: Data Lost When Print Or Exporting Report?

i have a asp.net project i want the user be able to select the fields from checkboxes near every filed in grid and also filter it by selecting the operator form dropdownlist and filter string from textbox in grid so i create a empty datatable in dataset and it have 7 columns i bind my crystal report to it and i use the following code for showing fields it works and i see the data but when i want to export or print it only the headers remain data disappeare here is my code:

protected void Button1_Click(object sender, EventArgs e)
{
MultiView1.ActiveViewIndex = 1;
//ReportDocument objRpt = new ReportDocument();
string reportPath = Server.MapPath("CrystalReport.rpt");

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