Just for a future reference, cache is invalidated automatically in a web farm environment when there are any changes (e.g. content, settings, media files) made on one of the servers.
If that does not work, it could be caused by 2 possible reasons:
Synchronization between the web farm servers is non-functional due incorrect configuration or licensing issues.
Web farm tasks have not been processed yet - especially when using "Update within request" mechanism, it might take some time till that happens.
More information about how web farms work in Kentico can be found in this blog post - Kentico 8 Technology – Web farm synchronization.