tab probelm in autopostback in textbox

<asp:TextBox ID="txtBlockName" MaxLength="15" runat="server" Text='<%#Eval("BlockName") %>'
                                                                    OnTextChanged="txtBlockName_TextChanged" AutoPostBack="true"></asp:TextBox>

protected void txtBlockName_TextChanged(object sender, EventArgs e)
        {
            try
            {
                TextBox txtBlockName = sender as TextBox;
                // FINDING THE PARENT CONTROL OF DROPDOWN INORDER TO BIND 
                RepeaterItem RepDropDownRow = ((RepeaterItem)txtBlockName.Parent);
                if (objBlockBusinessRule.GetBlockNameValidate(txtBlockName.Text, objBlockLibrary.BlockId))
                {
                    RepBlockEntry.Visible = true;
 
                    txtBlockName.Enabled = true;
                    //txtBlockName.Focus();
                   // txtBlockName.TabIndex = 1;
                    spError1.Visible = true;
                    lblError1.ForeColor = System.Drawing.Color.White;
                    spError1.Attributes.Add("class", "alertboxnotsavedForQuick");
                    lblError1.Text = "Block Name Already Exists";
                }                
            }
 
            catch (Exception ex)
            {
                objCommon.WriteStatus(ex.Source, "Error", ex.InnerException + "\r\n" + ex.StackTrace);
            }
        }