Random String Generator Creates Same String On Multiple Calls

I've build a random string generator but I'm having a problem whereby if I call the function multiple times say in a Page_Load method, the function returns the same string twice.here's the code

Public Class CustomStrings
''' <summary>'
''' Generates a Random String'
''' </summary>'
''' <param name="n">number of characters the method should generate</param>'
''' <param name="UseSpecial">should the method include special characters? IE: # ,$, !, etc.</param>'
''' <param name="SpecialOnly">should the method include only the special characters and excludes alpha numeric</param>'
''' <returns>a random string n characters long</returns>'

Public Function GenerateRandom(ByVal n As Integer, Optional ByVal UseSpecial As Boolean = True, Optional ByVal SpecialOnly As Boolean = False) As String.............