Caching & Wordpress

I'll try to keep this short and clear. Sorry if I failed. ;) I can not get caching working on one of my cPanel accounts and it's domains. I have multiple domains under one account, when I do a fresh Wordpress install, by default gives this response header: cache control: private, must-revalidate
It overrides any caching plug in I put in place. There is nothing in .htaccess that would tell the site not to cache. Then, when I try another account on the same server, which has only 1 domain, the response headers are ok and I can get caching plugins working. It seems like this problem is isolated to just one cPanel account, which has multiple domains under it. Any tips / ideas what I should be looking for to getting this fixed? edit: Webserver is Apache.