Theme overrides don't override modules.tpl template

I want to add classes to the markup of the subscribe block.

So I copied the file sites/all/modules/google_groups/google-groups-subscribe-form-block.tpl.php to sites/all/themes/customtheme/google-groups-subscribe-form-block.tpl.php.
I added block.tpl.php to my theme from /modules/system/block.tpl.php to sites/all/themes/customtheme/block.tpl.php.
I added changes to the google-groups-subscribe-form-block.tpl.php in my theme.

I used devel to clear all caches, - no text appeared, theme developer says the block is themed from the module - (see attached screenshot).

My theme is a subtheme of Blueprint, so I added my changed google-groups-subscribe-form-block.tpl.php to that theme & it still will not show up in the block output.

I have cleared the caches between the changes. As mentioned I also tried to use the changed google-groups-subscribe-form-block.tpl.php in the Acquia slate theme but it still failed to alter the html of the subscribe block.

I'm using Drupal 6.12 with 6.x-10 of Google Groups.

EDIT
I forgot to mention, other overrides work from my custom theme, such as node-event.tpl.php overrides my event nodes and block-user.tpl.php overrides the main navigation block.

Well here is some more info from the status page…

Drupal 6.12
File system Writable (public download method)
GD library bundled (2.0.34 compatible)
GD library bundled (2.0.34 compatible)
MySQL database 5.0.67
PHP 5.2.6
PHP memory limit 256M
PHP register globals Disabled
Unicode library PHP Mbstring Extension
Web server Apache/2.2.9 (Unix) mod_ssl/2.2.9 OpenSSL/0.9.7l DAV/2 PHP/5.2.6
jQuery Update 1.2.6

I really don't know why this fails for me, any ideas?

No I haven't figured this one out, sorry.
Are you using the Blueprint theme too?

If I understand you correctly Ryan, a fix would be…

changing line 118 of google_groups.module from

<?php
/**
 * Implementation of hook_theme()
 */
function google_groups_theme() {
  return array(
    'google_groups_block0_content' => array(
      'template' => 'google-groups-subscribe-form-block',
      'arguments' => array(),
    ),
  );
}
?>

…to this…

<?php
/**
 * Implementation of hook_theme()
 */
function google_groups_theme() {
  return array(
    'google_groups_subscribe_form_block' => array(
      'template' => 'google-groups-subscribe-form-block',
      'arguments' => array(),
    ),
  );
}
?>

Is that what you mean? I tried altering it in the module to see if it would pick up the the google-groups-subscribe-form-block.tpl.php in my theme but it didn't seem to work. I expect there are other things I need to change.

I think you logic is correct because I can now override the google-groups-subscribe-form-block by adding a 'google-groups-block0-content.tpl.php' to my theme. I don't think that is the correct way to do the override.

Thanks for looking into this,
Drew

I updated to the 6.x-2.0 version and it fixed the issue.

Thank you ryan_courtnage and dragonwise.