C# 2nd dropdown list cant retain the selected item

protected void Page_Load(object sender, EventArgs e)
  {
      if (Page.IsPostBack)
      {

          filldropdown(dllselection.SelectedValue);
          Code.Enabled = true;
          if(dllselection.SelectedValue=="")
          {
              Code.Enabled = false;
          }
      }
  }

<div class="form-group">
          <label for="inputEmail3" class="col-sm-2 control-label"  style="color:black" >Main Category</label>
          <div class="col-sm-3">
              <asp:DropDownList ID="dllselection" runat="server" CssClass="form-control" AutoPostBack="true" required>
                  <asp:ListItem Text="Please Select" Value=""></asp:ListItem>
                  <asp:ListItem Text="HR" Value="M_1"></asp:ListItem>
                  <asp:ListItem Text="IT" Value="M_2"></asp:ListItem>
                 <asp:ListItem Text="Maintenance" Value="M_3"></asp:ListItem>
              </asp:DropDownList>
          </div>
          </div>

        <div class="form-group">
         <label for="Training" style="color:black" class="col-sm-2 control-label">Sub Category</label>
          <div class="col-sm-3">
              <asp:DropDownList ID="Code" Enabled="false" onchange="javascript:return dropdown(this);" runat="server" CssClass="form-control"  ValidationGroup="G1" required></asp:DropDownList>
          </div>
          </div>

i keep fail to get dropdownlist value for 2nd dropdrownlist. the value i get is always 1st value and text.

everytime I select any item from the 2nd ddl it posts back and the first item would be selected instead the item that I select.