CodeIgniter disable PHP errors when the ENVIRONMENT has been set to production. So make sure you have set the ENVIRONMENT to development if you are still in development mode for debugging purposes.
Look for the following line in start of the index.php file.
define('ENVIRONMENT', 'development'); // if it is production then change to devlopment
Once you have done, if you still don't see the errors then you might need to call ini_set('display_errors', 1); a PHP method for displaying the errors if your PHP codnfiguration is defaulted not to show the error message in screen.