Issue: Consider a scenario where ISA administrator wants to cache a particular website but he is not able to cache it. He has created rule to cache the rule. Everything about the rule and the caching is configured properly. However responses from the web server are not being cached by the ISA server.
Data collection to resolve the issue
Took HTTPWatch trace while accessing the problem website on a client machine. You can check about httpwatch here http://www.httpwatch.com/
Data analysis and Conclusion
Following is the snapshot( httpwatch) of the header for the request for that problem website
As we can see in the header received section (received from webserver) cache-control has value of no-cache, which means that web server is configured not to let the client or proxy server cache its responses due to which ISA server was not able to cache the responses from the web server. Another variation to cache-control value is shown below which would also not let proxy server to cache the web server responses.
so if you see such behaviour, please check the cache-contol value of the web server response and look for the values mentioned.