Quicktags external functions file

I have added shortcode buttons to the WordPress tinyMCE text editor using the WordPress quicktags function.

This works fine when in functions.php, however I need it to be in an external functions file which I am including in functions.php. For some reason it does not work in my external functions file.

The external function file definitely works, sinceother functions work fine within it.

I have also removed all code from the external file except the quicktags function to see if there is a conflict but it still does not work.

Any ideas on how i can get this to work?