Custom Taxonomy not working

Not really, that looks like the right way to do it. You could pass debug => true to get a HTML comment with lots of debugging information. You should see which SQL query is being executed and also which taxonomies are allowed.

I think I figured it out. You have to also define the custom post type in the args. For example:

$args = array(
‘title’ => ‘ ‘,
‘post_type’ => array(‘post_type_a’, ‘post_type_b’),
‘taxonomy’ => array(‘tag_a’,’tag_b’)
);
do_utcw( $args );

That seems to have worked for me.

Yes, that should do it. Default post_type is post