The cache storing in the client can be controlled using HTTP headers or by having HTTP-EQUIV meta tags in the WML content. If both of the previous cache controls have been defined, the HTTP headers have a higher priority.
Within WML content you can use:
<meta http-equiv="Cache-Control" content="max-age=0"/>
or
<meta http-equiv="Cache-Control" content="no-cache"/>
Please try to use this "no-cache" if you have problems with "max-age".
Also make sure that your origin server or gateway doesn't cache your files.