Most likely you added this custom code to the top of your BPS root .htaccess file. One of the checks that occurs to check if a BPS .htaccess file is in use is to check the top of the root .htaccess file for the BPS version number.
Use BPS Custom Code and add your caching code in this Custom Code text box below, click the Save Root Custom Code button, go to the Security Modes page, click the Create secure.htaccess File button and activate Root Folder BulletProof Mode again.
CUSTOM CODE TOP PHP/PHP.INI HANDLER/CACHE CODE: Add php.ini handler and/or plugin cache code here
ExpiresDefault "access plus 30 days"
Also we have come up with some decent caching code in the link below, BUT this caching code has some conflicts with WP Super Cache .htaccess code (and probably with W3TC caching code). We are looking into that and will find a solution for this. Currently we are only using this caching code and not any caching plugins.