EF Include method doesn.t result

I create Asp.Net Mvc4 application with Code First EF. I create my model, and now I want get relational info from Database. I write get method for this info as follow:

public IQueryable<Invoice.Model.Invoice> Invoices 
{ 
     get 
       { 
         return context.Invoices.Include("InvoiceItems").Include("InvoiceStatus").Include("BankAccount").Include("TaxType").Include("Contract");
       } 
}

Then I use this properties in my operations, but relational info suach as InvoiceStatus return null,, How can fix this problem? Thanks in advance.