This problem is proberbly arising from using the dreaded F word, Frames. One work around to force a browser to reload everytime is to put in a document expiry meta tag, with an old date, any date will do even from start of this week. The browser will load the cached version from visitors pc, see an out of date version and download the "new" content.
put the following line in between your <head> tags
<META http-equiv="Expires" content="Mon, 07 Feb 2005 12:00:00 GMT">
It may be useful if you give us the websites in question then maybe another suggestion can be given.