It took me half a day to finally realize, prestashop was posting too much variables on the product form with my additional fields.
There wasn't any error, but the form just return back to the front page with no exception, no error logs...
just plain - "not working".
echo "keys: ".count(array_keys($_REQUEST));
i found 990 keys posted.
then i went into whm and change php.ini in /usr/local/lib
then i found the keys posted is 1800+ keys..