Entity Framework 6: Add to child collections - entity-framework

Question

I am using EF6 code first. I have an parent entity (Expense) which has collection of child entity (Tag). Entities are
public class Expense : Entity
{
//public Expense()
//{
// this.Tags = new HashSet<Tag>();
//}
public string Title { get; set; }
public ICollection<Tag> Tags { get; set; }
//public long TagId { get; set; }
public decimal Amount { get; set; }
public long ByUserId { get; set; }
public User ByUser { get; set; }
public long? ForUserId { get; set; }
public User ForUser { get; set; }
public DateTime ExpenseDate { get; set; }
}
public class Tag : Entity
{
public Tag()
{
this.Expenses = new HashSet<Expense>();
}
public string Name { get; set; }
public virtual ICollection<Expense> Expenses { get; set; }
public long OrganizationId { get; set; }
public Organization Organization { get; set; }
}
Configured as
public ExpenseConfiguration()
{
this.HasRequired(x => x.ByUser)
.WithMany()
.HasForeignKey(x => x.ByUserId)
.WillCascadeOnDelete(false);
this.HasOptional(x => x.ForUser)
.WithMany()
.HasForeignKey(x => x.ForUserId)
.WillCascadeOnDelete(false);
this.HasMany(x => x.Tags)
.WithMany(t => t.Expenses)
.Map(et => {
et.MapLeftKey("ExpenseId");
et.MapRightKey("TagId");
et.ToTable("tblExpenseTags");
});
}
public TagConfiguration()
{
this.HasRequired(x => x.Organization)
.WithMany()
.HasForeignKey(p => p.OrganizationId)
.WillCascadeOnDelete(false);
}
The way I am saving entity
var tags = new List<Tag>();
foreach (var item in expense.Tags)
{
var tag = _TagHandler.Get().Where(x => x.Id == item.Id).FirstOrDefault();
tags.Add(tag);
}
entity.Tags.Clear();
foreach (var item in tags)
{
expense.Tags.Add(item);
}
if (expense.Id == 0)
{
entity.CreatedDate = DateTime.UtcNow;
entity.UpdatedDate = DateTime.UtcNow;
updated = EntityRepository.Add(entity);
}
else
{
expense.UpdatedDate = DateTime.UtcNow;
updated = EntityRepository.Update(entity);
}
UnitOfWork.Commit();
When i Create Expense I add multiple tags to it and it save successfully in tblExpenseTags, but the problem is when I add new tags to Expense they are not saving into tblExpenseTags table. I used sql profiler to see what calls are going to SQL database and I don't see any insert call at the time of update expense.
Please let me know what wrong am I doing ?