AJAX :: AutoComplete On Textbox Not Showing List?

I'm having some difficulty adding an autocomplete extender to a textbox and getting it working.

I've read through the samples and did a quick search on here, but cannot work out why it isn't working.

In a nutshell...
I've added a textbox onto a page, and added the autocomplete extender along with the AutoComplete page method.

Page

[Code]....

Code Behind

[System.Web.Services.WebMethodAttribute(), System.Web.Script.Services.ScriptMethodAttribute()]
public string[] GetCompletionList(string prefixText, int count, string contextKey)
{
StaffADBLL staff = new StaffADBLL();
DataTable dtStaff = staff.FindStaff(prefixText);
string[] items = new string[dtStaff.Rows.Count];
int i = 0;
foreach (DataRow row in dtStaff.Rows)
{
items.SetValue(row["displayName"].ToString(), i);
i++;
}
return items;
}

I've even tried adding an asmx file, and that doesn't work either.

I get no errors and when I start typing nothing happens, it doesn't even produce a list after 3 characters.
I tried to debug and it doesn't debug through the code, skips it completely.