Label.Text not getting cleared/overwritten

Ihave a grid view, a label and a button functionality in aspx with. I am displaying a msg in the label if there is an exception. I am re-writing the label text when the button is clicked again which should clear the text in the label, but this is not happening. I tried to clear/overwrite blank at multiple places as you can see in the code, but It does not get cleared. Please suggest how I can clear the label text.
aspx:
<div>
    <h2 id="h2Concurrent">CONCURRENT CASES <img src="images/arrowDown.png" /></h2>
    <asp:GridView ID="gvConcurrentCases" runat="server"
            EmptyDataText="No Concurrent cases Found"  
        AutoGenerateColumns="False" EnableViewState = "true" >
        < ...