[Hummingbird Pro] Browser Caching is not working properly:

I am having problem setting up browser caching on my site, getting the following error...

- Your server may not have the "expires" module enabled (mod_expires for Apache, ngx_http_headers_module for NGINX)
- Another plugin may be interfering with the configuration

I have verified with the server that the mod above is set up, so presuming it must be to do with another plugin.

Is there a certain type of plugin that may affect it? I am not using any plugins that I don't use on other sites where I have this working fine.