node_mass_update will not update field
I've never used (or even seen) that function before, but following the trail down to _node_mass_update_helper(), I think you need to use a format like this:

$updates = array('field_votes' =&gt; array(0 =&gt; array('value' =&gt; $votes)));


That array format is the standard way to update CCK fields in Drupal 6, and since the helper function does this:

foreach ($updates as $name =&gt; $value) {
  $node-&gt;$name = $value;
}


immediately before calling node_save(), I think that's what the problem is.