context.Entry(entity).State = EntityState.Modified; won't update my record.

I have another project that uses the same way like context.Entry(entity).State = EntityState.Modified; to update a record and it works but on this project is does not. How can I fix this? and why is it happening?

Here is my code: 

using (Exercise2Entities db = new Exercise2Entities())
                {

                    User userAcc = null;
                    if (ddlUsers.SelectedValue != "")
                    {
                        int userId = Convert.ToInt32(ddlUsers.SelectedValue);
                        //userAcc = db.Users.SingleOrDefault(u => u.Id == userId);
                        userAcc = db.Users.Find(userId);
                        lblError.Text = "Reached here";
                        //userAcc.Id = userId;
                        userAcc.Name = tbName.Text;
                        userAcc.Age = int.Parse(tbAge.Text);
                        userAcc.IsStudent = cbIsStudent.Checked.ToString();
                        userAcc.Date = DateTime.Parse(tbBirthdate.Text);


                        //db.Entry(userAcc).CurrentValues.SetValues(newUser);
                        db.Entry(userAcc).State = EntityState.Modified;
                    }
                    db.SaveChanges();

                    // reload drop down list
                    LoadUserDropDownList();

                    // select the one the user just saved
                    ddlUsers.ClearSelection();
                    ddlUsers.Items.FindByValue(userAcc.Id.ToString()).Selected = true;

                    LoadListViewWithUserInfo();

                }