Form submit is not firing on large form I have an old D6 site. I created a form with hook_form with very many fields (197 rows * 6 columns = 1182 fields). It works good and form submit works properly. 

But when I add a new, 7th column to the form and the fields number becomes 1379 the form submit is not firing.

I do not see any errors or warnings, php memoty limit is set to 256M and post max size too. But submit handler simple does not fire.

What can be a cause?
Increase max_input_vars in your PHP environment as Clive suggested in the comment.

Another indication of this problem is the form validate handler won't trigger either. Unless you have E_WARNING set in the php.ini error_reporting setting the form will silently fail and run the form hook again.