Can't seem to get attachment_fields_to_edit to fire on live server

Hi, I’m having trouble getting attachment_fields_to_edit to fire on my live server. It works as it should locally. The live install is using multisite…would that cause a problem?

A simple example that I can’t get to work on live server…

add_filter( 'attachment_fields_to_edit', 'test_filter', 50, 2 );
function test_filter(){
	echo 'testing filter<br/>testing filter';
}

I’ve narrowed my problem down to this, but I’m not sure how to debug this. Any advice or guidance is much appreciated!