Plugin hook not working on specific WordPress installation – how to troubleshoot?

Found the answer. Apparently plugins don’t work on get_the_content(), so I used the output buffering example below. I found it here

ob_start();
the_content();
$newContent = ob_get_clean();

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)