Radcombobox selected Index changed event button click

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VB

1.I'm having a two radcombobox with parent child combination.
2.First radcombobox acts as a Parent and Second radcombobox acts as a child.Second radcombobox will bind depends upon the First radcombobox selectedvalue
3.I'm using the SelectedindexChanged event to bind the Second radcombobox.
3.I'm also having a Button Field Event.
4.Now my problem is
when i'm Selecting the First Combobox and Clicking the Button the Second radcombobox is not binded...How to solve this type of issues..

First Combobox:

<telerik:RadComboBox ID="cbxFundingDeptName" Height="300PX" EmptyMessage="Please Select" CssClass="rad_ddl" Localization-CheckAllString="Select All"
CheckBoxes="true" runat="server"  OnClientDropDownClosed="cbxFundingAgencyClosed" OnSelectedIndexChanged="cbxFundingDeptName_SelectedIndexChanged"
CheckedItemsTexts="DisplayAllInInput" EnableCheckAllItemsCheckBox="true"  AllowCustomtext="True"></telerik:RadComboBox>

Second Combobox:

<telerik:RadComboBox Height="300PX" ID="cbxContractingOfficeDeptName" EmptyMessage="Please Select" Localization-CheckAllString="Select All"
CssClass="rad_ddl" CheckBoxes="true" runat="server" OnClientDropDownClosed="cbxProcuringAgencyClosed"
CheckedItemsTexts="DisplayAllInInput" EnableCheckAllItemsCheckBox="true"  AllowCustomtext="True"></telerik:RadComboBox>

Selected Index changed Event:

Protected Sub cbxFundingAgency_SelectedIndexChanged(ByVal sender As Object, ByVal e As Telerik.Web.UI.RadComboBoxSelectedIndexChangedEventArgs)
        PopulateFundingAccount()
    End Sub

How to solve this .......