Drupal 7 custom form submit handler is not being called
I had this same problem trying to edit commerce_product form. For some reason the submit handlers are registered under actual submit button and custom handlers added to the #submit -element weren't registered.

$form['#submit'][] = 'my_custom_handler';
$form['actions']['submit']['#submit'][] = 'my_custom_handler';


So when I added that second line to register my custom handler to button and atleast it works.