Why My ModelState.IsValid doesn't return true In Asp.Net MVC4?

Question

I have a controller which looks like;

[HttpPost]
[Authorize(Roles = "Admin")]
public ActionResult ProjectAdd(PortfolioViewModel model, int[] categories, HttpPostedFileBase thumbnail, HttpPostedFileBase image)
{
    model.ProjectImage = System.IO.Path.GetFileName(image.FileName);
    model.ProjectThubmnail = System.IO.Path.GetFileName(thumbnail.FileName);
    using (PortfolioManager pm = new PortfolioManager())
    {
        using (CategoryManager cm = new CategoryManager())
        {
            if (ModelState.IsValid)
            {
                bool status = pm.AddNewProject(model, categories);
            }
            ViewBag.Categories = cm.GetAllCategories();
            ViewBag.ProjectsList = pm.GetAllProjects();
        }
    }
    return View(model);
}

My View is;

@using (Html.BeginForm("projectAdd", "home", FormMethod.Post, new { enctype = "multipart/form-data" }))
{
    @Html.ValidationSummary(true)

    <fieldset>
        <legend>Add New Project</legend>

        <div class="editor-label">
            @Html.LabelFor(model => model.ProjectHeading)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.ProjectHeading)
            @Html.ValidationMessageFor(model => model.ProjectHeading)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.ProjecctUrl)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.ProjecctUrl)
            @Html.ValidationMessageFor(model => model.ProjecctUrl)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.ProjectLongDescription)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.ProjectLongDescription)
            @Html.ValidationMessageFor(model => model.ProjectLongDescription)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.PromoFront)
        </div>
        @Html.EditorFor(model => model.PromoFront)
        @Html.ValidationMessageFor(model => model.PromoFront)

        <div class="editor-label">
            <label for="thumbnail">Thumbnail</label>
        </div>
        <div class="editor-field">
            <input type="file" name="thumbnail" id="thumbnail" /> 
        </div>
        <div class="editor-label">
            <label for="image">Image</label>
        </div>
        <div class="editor-field">
            <input type="file" name="image" id="image" /> 
        </div>
        <div class="editor-label">
            <label for="categories">Categories</label>
        </div>
        @foreach (var c in categories)
        {
            <input type="checkbox" name="categories" value="@c.CategoryId">
            @c.CategoryName
        }
        <p>
            <input type="submit" value="Create" class="submit" />
        </p>
    </fieldset>
}

When I try this code, The ModeState.IsValid property becomes false (I saw through debugging). However, when I remove ModeState.IsValid, the insertation is done successfully and everything works exactly what I want.
I need ModeState.IsValid property for validating my view.
Updated: My viewmodel is;

[Key]
public int ProjectId { get; set; }
[Required(ErrorMessage="Please enter project heading")]
public string ProjectHeading { get; set; }
[Required(ErrorMessage = "Please enter project Url")]
public string ProjecctUrl { get; set; }
[Required(ErrorMessage = "Please enter project description")]
public string ProjectLongDescription { get; set; }
public string ProjectShortDescription
{
    get
    {
        var text = ProjectLongDescription;
        if (text.Length > ApplicationConfiguration.ProjectShortDescriptionLength)
        {
            text = text.Remove(ApplicationConfiguration.ProjectShortDescriptionLength);
            text += "...";
        }
        return text;
    }
}
public bool PromoFront { get; set; }
[Required(ErrorMessage = "You must sepcify a thumbnail")]
public string ProjectThubmnail { get; set; }
[Required(ErrorMessage = "You must select an image")]
public string ProjectImage { get; set; }
public int CategoryId { get; set; }
public IEnumerable<Category> Categories { get; set; }

Updated 2: I found the error. the problem is

{System.InvalidOperationException: The parameter conversion from type 'System.String' to type 'PortfolioMVC4.Models.Category' failed because no type converter can convert between these types.
   at System.Web.Mvc.ValueProviderResult.ConvertSimpleType(CultureInfo culture, Object value, Type destinationType)
   at System.Web.Mvc.ValueProviderResult.UnwrapPossibleArrayType(CultureInfo culture, Object value, Type destinationType)
   at System.Web.Mvc.ValueProviderResult.ConvertTo(Type type, CultureInfo culture)
   at System.Web.Mvc.DefaultModelBinder.ConvertProviderResult(ModelStateDictionary modelState, String modelStateKey, ValueProviderResult valueProviderResult, Type destinationType)}

Answer 1

When in debug, check the ModelState for errors. It's a key/value dictionary with all the properties required to make the model valid. If you check the Values-property you can find the value with the Errors-list that is not empty and see what the error is.

Or add this line of code in the action method to get all the errors for the model:

var errors = ModelState.Where(v => v.Value.Errors.Any());