gform_paypal_fulfillment Not Firing?

Can someone help me diagnose an issue that I'm having with gform_paypal_fulfillment? Here's the scenario:

1. test registration form sends data to paypal sandbox
2. sandbox appears to be returning data correctly, because user is registered according to Add User Registration plugin, and database contains transaction IDs, "approved" status, etc.
3. At this point, I'd expect my basic function to fire and echo the text on the form's confirmation screen. The basic function from functions.php, which was essentially copied from the documentation:

add_action("gform_paypal_fulfillment", "process_order", 10, 2);
function process_order($entry, $config, $transaction_id, $amount) {
echo 'hello from the paypal fulfillment function';
}

4. ...But I get no echo.

What other info can I provide to help with diagnosis? I've got several other GF hooks that are working as expected.

Thanks!

Thanks, Carl. So if I can't see the echo, what else can I do (besides blind testing) to see and verify the variables that I'm trying to manipulate?

Is gform_user_registered still valid at this stage?

Will gform_user_registered not echo either?

I ask because I was using this hook successfully (with echos) prior to adding the Paypal component. Now, I get no echos from it, and none of my functionality is working. Does it still have access to $user_id? (I would think it would.)

Thanks for the speedy replies, Carl. I really appreciate it.