C# - Cache Is Not Invalidated After Table Update Using The SqlCacheDependency?

I have been trying to get SqlCacheDependency working. I think I have everything set up correctly, but when I update the table, the item in the Cache isn't invalidated.

I enabled the Service Broker for the Sandbox database.I have placed the following code in the Global.asax file. I also restart IIS to make sure it is called.

void Application_Start(object sender, EventArgs e) {
SqlDependency.Start(ConfigurationManager.ConnectionStrings["SandboxConnectionString"].ConnectionString);
}

I have placed this entry in the web.config file:

<system.web>
<caching>
<sqlCacheDependency enabled="true" pollTime="10000">[code]....

I call this code to put the item into the cache:

protected void CacheDataSetButton_Click(object sender, EventArgs e) {
using (SqlConnection sqlConnection = new SqlConnection(ConfigurationManager.ConnectionStrings["SandboxConnectionString"].ConnectionString))[code].....

Then I bind the GridView with this code:

protected void BindGridViewButton_Click(object sender, EventArgs e) {
if (Cache["Pets"] != null) {
GridView1.DataSource = Cache["Pets"] as DataSet;
GridView1.DataBind();
}
}

Between attempts to DataBind the GridView, I change the table's values expecting it to invalidate the Cache["Pets"] item, but it seems to stay in the Cache indefinitely.