browser caching for static subdomain

Not sure this is the right forum, but hope someone can help me…

I have a www domain and a static (sub)domain that’s on a virtual host (so no seperate fysical location). All images and other static content are served from the static (sub)domain. I checked and it works.

Now I try to leverage browser caching, via htaccess. It works for the www domain, but not for the static (sub)domain.

Anyone knows how to solve this?