There are caveats when enabling preloading, one of them being that it should be enabled via a php.ini file. Enabling it with a php-fpm pool configuration won't work, for instance, since preloading is global and not per-pool. To make sure that you successfully enabled preloading, you should check for a preload_statistics key in the output of opcache_get_status(). There should already be an opcache_statistics key, but that's something else entirely.