I get inconsistent results trying to use "FormsAuthentication.GetRedirectUrl("?", True).ToString()

I am trying to grab the redirection URL and place it in a string.  such as
xtrText = "FormsAuthentication.GetRedirectUrl("?", True).ToString()
I get inconsistent results with it.  It works fine locally or on the LAN, but on a Wan, sometimes it does work and somet times it doesn't.  For example, I  have a three devices, a Motorla Zoom, an IPAD, and a LapTop using exactly the same wireless connection, I have cleared out the cache in all devices, and when I access this Web App on the internet, it doesn't work for the Zoom, or the LapTp, but does for the IPAD.  In otherwords, strText shows to be an empty string for the Zoom and Laptop, but the IPAD shows the correct redirect url.
Any ideas?
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