ASP.Net MVC 3 Retrieve CheckBoxList Values

I am developing an ASP.Net MVC 3 Web application and I am having difficulties retrieving the selected checkbox values within the HttpPost method in my Controller. Hopefully someone can help.

I have 2 ViewModels

public class ViewModelShiftSubSpecialties
{
    public IEnumerable<ViewModelCheckBox> SpecialtyList { get; set; }
}

public class ViewModelCheckBox
{
    public string Id { get; set; }
    public string Name { get; set; }
    public bool Checked { get; set; }
    public string Specialty { get; set; }
}

And a partial View I use as an EditorTemplate

@model Locum.UI.ViewModels.ViewModelCheckBox

@Html.HiddenFor(x => x.Id)        
@Html.CheckBoxFor(x => x.Checked)
@Html.LabelFor(x => x.Name, Model.Name)<br />

In my View I create the checkboxes under two headings, Medicine and Surgery

<h3>Medicine</h3>
foreach (var sub in Model.SpecialtyList)
{
if (sub.Specialty.Equals("Medicine"))
 {
    @Html.EditorFor(m => sub)
 }
}
<h3>Surgery</h3>

foreach (var sub in Model.SpecialtyList)
{
if (sub.Specialty.Equals("Surgery"))
 {
    @Html.EditorFor(m => sub)
 }

}

And then in my HttpPost Controller I try to get the values of the selected checkboxes, but mode.SpecialtyList is always Null

 [HttpPost]
    public ActionResult AssignSubSpecialties(ViewModelShiftSubSpecialties model)
    {
        foreach (var item in model.SpecialtyList)
        {
            if (item.Checked)
            {
                //do some logic
            }
        }

        return View();
    }

Does anyone know why model.SpecialtyList is always Null?

Any help is much appreciated.

Thanks.