Problem with 'save_post' hook not running

I have a custom post type, with a custom meta box which contains a number of input fields.

The meta box is displaying fine but when completing all the fields the data is not saving, in fact, the function I call with the save_posthook does not appear to be running at all…

I use the register_meta_box_cb to define my function which adds the meta box for me. I then use the following in my attempt to save the custom data…

add_action( 'save_post', 'my_save_function', 10, 2 );
function my_save_function( $post_id, $post )	{
foreach( $_POST as $meta_key => $new_meta_value )	{
if( substr( $meta_key, 0, 6 ) == 'venue_' )	{ // This is verified and correct
update_post_meta( $post_id, $meta_key, $new_meta_value );
}
}
}

Been staring at this and searching the codex and Google for a couple of hours now with no progress… Whatever I put in that save function just does not execute. I have tried die() and error_log at the very top of the function to ensure my checks are not the problem… but nothing. (the function above is a cut down version to what I really have)