Shot in the dark here, but…
It is entirely possible the plugin with the do_action definition is hooked before the theme is processed.
Find out where the do_action is defined, and find out when it is being hooked.
You might need to hook to the function that the do_action definition is hooked too, and THEN hook to that action definition.
Example:
Open the plugin file that has the do_action definition you are trying to hook to with your custom function.
Look to see if the do_action definition resides within a plugin function.
If so, look through the plugin to find an add_action() reference of that particular function name containing the do_action definition.
Note down what that hook is.
Now, you know when WordPress calls that plugin’s function containing the do_action definition.
So now in your theme functions.php file, you might have something similar to the following code:
/**
* This is the WordPress action the plugin's do_action function definition is
* hooked to.
*
* Example: This hook could be anything. I'm not saying the hook will be: "plugins_loaded"
* for sure, but if it was "plugins_loaded"... After WordPress loads and instantiates all
* of it's activated plugins, WordPress will fire the plugin's function containing the
* plugin's do_action definition (As long as the plugin you are trying to work with is
* activated). So you're getting on the same level as the plugin when it needs WordPress to
* execute this particular defined custom action and telling WordPress that your theme function
* needs to be on that same level as well, before it can hook to your plugin's do_action reference.
*/
add_action('plugins_loaded', 'wpse_setup_theme');
function wpse_setup_theme(){
/**
* This your function that you want fired then the do_action is executed.
*
* Example: If the plugin file has a function named osmosis_jones() and
* inside osmosis_jones(), there is a do_action() reference. Note down
* the do_action tag name inside the osmosis_jones() function.
*/
add_action('the_plugin_do_action_tag_name', 'wpse_display_theme_header');
}
function wpse_display_theme_header(){
echo 'THEME HEADER HERE!';
}