gform_validation filters don't seem to be picking up form

I'm not sure Gravity Forms is picking up my custom validation. I've added it to the functions.php file of the theme at the end of the file. I'm not getting any PHP errors but it doesn't seem to be working. For the validations filter function I'm using the example and have only modified the items below. I tested out the actual validation function (is_serial) separately and it functions correctly. I have no idea why it's not picking up the form data or applying the filter.

add_filter('gform_validation_13', 'lx_validate_serial', 10, 2);

function lx_validate_serial($validation_result) {

... same as example code ...

// If the field does not have our designated CSS class, skip it
if(strpos($field['cssClass'], 'lx_serial') === false)

... same as example code ...

// Make a call to your validation function to validate the value
$is_valid = is_serial($field_value);

... same as example code ...

$field['validation_message'] = 'The serial number you have entered is not valid.  Please verify the serial number on your device';

... same as example code ...

}

Thanks, I saw it belatedly. Then I went back and modified the post since that is where I put it and it still doesn't seem to be applying the filter.

I would have deleted the post and started over but that wasn't an option.