Cache looks like never been made in ASP.NET SQL - asp.net

Question

I'm developing website with query notification using SqlCacheDependency. When the website loads for the first time, if there is any cache, it retrieves the data from the cache while the opposite, if there is no cache, it retrieves data from database and then insert the cache.
When the user inserts data, and clicks a button, the website stores the data to the database, removes the cache, so when the page redirects to itself, there is no cache, which makes the website should retrieve the new data.
Here is the following code:
protected void Page_Load(object sender, EventArgs e)
{
if (IsPostBack)
return;
this.BindGrid();
System.Diagnostics.Debug.WriteLine(Cache.Count);
//cache counts : -4; I don't even know why
}
protected void btnSave_Click(object sender, EventArgs e)
{
this.Insert(this.txtTitle.Text, Convert.ToInt32(this.txtDescription.Text));
//insert just function to update the database
Cache.Remove("Messages");
this.BindGrid();
}
private void BindGrid()
{
DataTable dtMessages = (DataTable)Cache.Get("Messages");
if (dtMessages == null)
{
dtMessages = this.LoadMessages();
lblDate.Text = string.Format("Last retrieved DateTime : {0}", System.DateTime.Now);
}
else
{
lblDate.Text = "Data Retrieved from Cache";
}
grdMessages.DataSource = dtMessages;
grdMessages.DataBind();
}
private DataTable LoadMessages()
{
DataTable dtMessages = new DataTable();
dtMessages = new DataTable();
using (SqlConnection connection = new SqlConnection(this.ConnectionString))
{
SqlCommand command = new SqlCommand("select * from blablabla", connection);
SqlCacheDependency dependency = new SqlCacheDependency(command);
if (connection.State == ConnectionState.Closed)
connection.Open();
dtMessages.Load(command.ExecuteReader(CommandBehavior.CloseConnection));
if (Cache.Get("Messages") == null)
{
Cache.Insert("Messages", dtMessages, dependency);
}
}
return dtMessages;
}
The problem is, looks like the cache hasn't been made/inserted. Because the value of Cache.Count is negative number. And whenever I just refresh the page (where the page supposed to retrieve data from cache), the page retrieves the data from database.
What's wrong and what is the solution? There are two important things to know:
Page only retrieves from database when there is no cache (cache'll be removed when user inserts data)
Page retrieves from cache when there is no any updated/new data from user (e.g. user just refreshes the page, page retrieves from cache)
Please help. Thank you for your time and attention.