You are having problems with the browser caching the XML file (you can test this by clearing your browser cache after you have made a change to the XML).
Since you can't use meta tags in an XML data file, you need a different solution to control caching. You can use the .htaccess based methods described
here, provided your Apache server has the required modules loaded.
If you can't do that, make a PHP page that sets the 'text/xml" content type along with the desired cache control headers and simply dumps your XML as output. Reference that instead of the XML file directly.
The third trick people use for this is to load the XML island using Javascript and append a random number query string, so that the request looks like "myfile.xml?7543758", the number being different every time. This is probably the least clean, most kludge way -- it doesn't stop caching, just tricks the browser into not using the cached copies, thus polluting the browser cache.
