Gridview not sorting - asp.net

Question

I've come across this strange error and it's been bugging me for hours now.
I've got some LinkButtons on my screen and depending on which button the user chooses a label's text is set, a GridView is displayed and the Datasource of this GridView is assigned based on the text of the label.
This all works fine and dandy. However the problem arises when I want to sort this GridView.
At the moment, nothing happens. When I attempt to sort a column the page just refreshes and I'm left with the same unsorted mess in my GridView.
I've put my application into debug mode with breakpoints along the way and noticed that when I get to this step (the full code can be seen at the bottom of this post) :
senderGridView.SortExpression = button.CommandArgument
the senderGridView.SortExpression is "" so for some reason it's not passing the sort expression.
Hopefully somebody can enlighten me as to why my grid isn't sorting as I'm sure it's just a stupid mistake somewhere but any help would be appreciated, thanks.
Code that you may require...
My sub that sets the sort images when a row is created can be seen below:
Protected Sub GridViewSortImages(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
Dim senderGridView As GridView = CType(sender, GridView)
'Loop through each cell in header row
For Each cell As TableCell In e.Row.Cells
If cell.HasControls Then
Dim button As LinkButton = TryCast((cell.Controls(0)), LinkButton)
Dim gv As New HtmlGenericControl("div")
Dim lnkName As New Label()
'Test cell to see if a link button exists (i.e. Allow Sorting = True)
If Not (button Is Nothing) Then
'Create new label and image and set to standard unsorted view
lnkName.Text = button.Text
Dim image As New System.Web.UI.WebControls.Image
image.ImageUrl = "images/sort-1x1.png"
image.ToolTip = "Sort"
image.AlternateText = "Sort"
'Test to see if grid is already sorted, and apply relevant image & tooltip
If senderGridView.SortExpression = button.CommandArgument Then
If senderGridView.SortDirection = SortDirection.Ascending Then
image.ImageUrl = "images/sort-asc.png"
image.ToolTip = "Sort Descending"
image.AlternateText = "Sort Descending"
Else
image.ImageUrl = "images/sort-desc.png"
image.ToolTip = "Sort Ascending"
image.AlternateText = "Sort Ascending"
End If
End If
'Add label and image to new div
gv.Controls.Add(lnkName)
gv.Controls.Add(image)
'Replace original column header with new div
button.Controls.Add(gv)
End If
End If
Next
End Sub
My RowCreated sub for the GridView...
Protected Sub grvHomeRisk_RowCreated(sender As Object, e As GridViewRowEventArgs) Handles grvHomeRisk.RowCreated
If Not (e.Row Is Nothing) AndAlso e.Row.RowType = DataControlRowType.Header Then
GridViewSortImages(sender, e) 'Call sort code
End If
End Sub
My Sorting sub...
Protected Sub grvHomeRisk_Sorting(sender As Object, e As GridViewSortEventArgs) Handles grvHomeRisk.Sorting
Select Case lblBreachHeaderb.Text
Case "Current Risks"
grvHomeRisk.DataSource = SQLDS_ListCurrentRisk
Case "All Risks Overdue"
grvHomeRisk.DataSource = SQLDS_ListAllRiskOverdue
Case "My Risks"
grvHomeRisk.DataSource = SQLDS_ListMyRisk
Case "My Risks Overdue"
grvHomeRisk.DataSource = SQLDS_ListRiskOverdue
Case "Risks Requested to Score & Re-Score"
grvHomeRisk.DataSource = SQLDS_ListScores
Case "Risks Requested to Score"
grvHomeRisk.DataSource = SQLDS_ListRiskScores
Case "Risks Requested to Re-Score"
grvHomeRisk.DataSource = SQLDS_ListRiskReScores
End Select
grvHomeRisk.DataBind()
End Sub
My GridView...
<asp:GridView ID="grvHomeRisk" runat="server" AutoGenerateColumns="false" AllowSorting="true"
CssClass="GridMain" UseAccessibleHeader="false"
ForeColor="#333333" GridLines="None" Width="780px" BorderWidth="0px"
AllowPaging="true" PageSize="5" CellPadding="3" DataKeyNames="IDRISK">
<Columns>
<asp:CommandField ButtonType="Image" SelectText="View Risk" ShowSelectButton="True" SelectImageUrl="~/Images/button-select1.png" />
<asp:BoundField DataField="TXRISKSUMMARY" HeaderText="Risk" ReadOnly="True" />
<asp:BoundField DataField="TSRISKSTART" HeaderText="Start Date" ReadOnly="True" DataFormatString="{0:dd MMM yyyy}" />
<asp:BoundField DataField="TSRISKREVIEW" HeaderText="Review Date" SortExpression="TSRISKREVIEW" ReadOnly="True" DataFormatString="{0:dd MMM yyyy}" />
<asp:BoundField DataField="TXUSER_NAME_O" HeaderText="Owner" SortExpression="TXUSER_NAME_O" ReadOnly="True" />
<asp:BoundField DataField="TXDESCRIPTION" HeaderText="Risk Element" SortExpression="TXDESCRIPTION" ReadOnly="True" />
<asp:BoundField DataField="TSDATMOD" HeaderText="Date Modified" ReadOnly="True" DataFormatString="{0:dd MMM yyyy}" />
<asp:BoundField DataField="TXUSER_NAME_M" HeaderText="Modified By" ReadOnly="True" />
</Columns>
<PagerTemplate>
<table>
<tr>
<td>
<asp:Button ID="FirstButton" runat="server" CommandArgument="First" CommandName="Page" Text="<<" CssClass="buttongrid" ToolTip="First" OnClientClick="needToConfirm = false;" />
<asp:Button ID="PrevButton" runat="server" CommandArgument="Prev" CommandName="Page" Text="Previous" CssClass="button" ToolTip="Previous" OnClientClick="needToConfirm = false;" />
</td>
<td>
<asp:Label ID="lblPageCount" runat="server" CssClass="TextA12Bold" Visible="false"></asp:Label>
</td>
<td>
<asp:Button ID="NextButton" runat="server" CommandArgument="Next" CommandName="Page" Text="Next" CssClass="button" ToolTip="Next" OnClientClick="needToConfirm = false;" />
<asp:Button ID="LastButton" runat="server" CommandArgument="Last" CommandName="Page" Text=">>" CssClass="buttongrid" ToolTip="Last" OnClientClick="needToConfirm = false;" />
</td>
</tr>
</table>
</PagerTemplate>
</asp:GridView>