Drupal hook_preprocess_hook not working
Nealv, the preprocess function can only be implemented for the base name, not for the additional suggestions you used for creating your tpl.php file.
The suggestions for alternate template files are added in the preprocess functions themselves.

See https://drupal.org/node/223440 - These naming suggestions are set from preprocess functions.

So call THEMENAME_preprocess_webform_form and check if it's the right form to alter within the function itself.