Here are a few things to check/try:
(1) Please try upgrading your gallery to the current version of SimpleViewer-Pro (v2.3.2).
Instructions for downloading the latest version and upgrading existing galleries can be found here.
(2) Make sure that your 'gallery-xml.php' file sends the correct header for an XML file by using the following code:
(3) There is whitespace before the XML declaration (<?xml version="1.0" encoding="UTF-8"?>) in your dynamically-generated XML file. Make sure that the XML declaration is output at the very top of the document.
(4) Try clearing your browser's cache before reloading your gallery to ensure that your browser is fetching and using the most recent files from your web server.
(5) If you are viewing your gallery over a 3G connection, please see this FAQ for a solution.
Why can't I view my gallery on a 3G mobile connection?
Hopefully one of the suggestions above will help.
SimpleViewer Support Team