OnRowCommand 事件有时不会触发

我有带有ButtonField的GridView：

 <asp:GridView ID="activitygrid" runat="server" CssClass="EU_DataTable" class="ui-body-d"   OnRowCommand="activitygrid_RowCommand1"  AutoGenerateColumns="False" OnSelectedIndexChanged="activitygrid_SelectedIndexChanged" ShowHeaderWhenEmpty="True" >

      <asp:ButtonField CommandName="cmdRegister" Text="register now"  ControlStyle-CssClass="table-stripe" HeaderText="register" ButtonType="Button" ControlStyle-Height="20px" >
                         <ControlStyle CssClass="table-stripe"></ControlStyle>
                     </asp:ButtonField>

页面加载代码：

 protected void Page_Load(object sender, EventArgs e)
        {                     
                if (!Page.IsPostBack)
                {
                    BindData();
                }
                else
                {
                    BindDataSecondTime();
                }            
        }

行命令代码：

protected void activitygrid_RowCommand1(object sender, GridViewCommandEventArgs e)
        {
            int index = Convert.ToInt32(e.CommandArgument);

            if (e.CommandName == "cmdRegister")
            {
                registerActivities.Add(relActivities[index]);
                //Update the sum
                CurrentSum += relActivities[index].ActivityPrice;
                lblSum.Text = CurrentSum.ToString();
                selectedactivitygrid.DataSource = registerActivities;
                selectedactivitygrid.DataBind();
                relActivities.Remove(relActivities[index]);
                activitygrid.DataSource = relActivities;
                activitygrid.DataBind();
                }
            }

问题是当我单击按钮时

，有时会触发事件（通常是当我单击按钮中间时），有时不会.

请帮忙。