Try updating you image button html somelike below
<asp:ImageButton runat="server" ID="imgCPTICD"
ImageUrl="~/Images/Plus.JPG" CommandName="Show"
CommandArgument="<%#((GridViewRow)Container).RowIndex%>"/>
If you are using Index only for finding the Image button then you can replace the code like below
You can directly take the sender as the Show(+) button
Current Code
ImageButton imgBtn = (ImageButton)grvRecords.Rows[RowIndex].FindControl("imgCPTICD");
New Code
ImageButton imgBtn = (ImageButton)sender;