Records in ICollection navigation property are not being written

I'm trying to include items in a property of type list but I'm not able to include the records in the database.

Firstly I have the following class models:

public class Frota
{
    public Frota() {
        Veiculos = new List<Veiculos>();
    }

    public string Id { get; set; }
    public virutal ICollection<Veiculo> Veiculos { get; set; }
}

public class Veiculo
{
    public string Id { get; set; }
    public string Nome { get; set; }   
}

Mapping:

public class FrotaMap : EntityTypeConfiguration<Frota>
{
    ...
    HasMany(x => x.Veiculos)
        .WithMany()
        .Map(x =>
        {
            x.MapLeftKey("FrotaId");
            x.MapRightKey("VeiculoId");
            x.ToTable("FrotaVeiculos");
        });
}

I run a two-step process where I create a Batch and then add the vehicles:

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<ActionResult> Edit([Bind(Include="...")] FrotaViewModel model)
{
    if (ModelState.IsValid)
    {
        var frota = await Context.Frotas
            .Include(x => x.Veiculos)
            .SingleOrDefault(x => x.Id == model.Id);

        foreach (var veiculo in frota.Veiculos)
            frota.Veiculos.Remove(veiculo);
        frota.Veiculos.Clear();

        foreach (var veiculo in model.Veiculos)
            frota.Veiculos.Add(veiculo);

        Context.Entry(frota).State = EntityState.Modified;
        await Context.SaveChangesAsync();
    }
    return RedirectToAction("Edit", new { id = model.Id })
}

But Veiculos is not added to the database and I can not understand why.

What is missing or wrong in this code?