Count of seats in classroom

I have a registration-form an a classes-form. The classes-form has different classes and defines, how many seats are available. The registration-form implements the classes-form and should decount by every registration the seats in the class-table, so I know, when the class is fully booked. For this decount I became some years ago a code-snipped.

Now I have a problem in my registration form. It worked last year, but this year it always copies the max amount of seats from the class-form to the reservation-form and the amount of available seats in the class-form stays the same.

Here the Code, that worked last year perfect:

add_action('frm_after_create_entry', 'after_entry_created', 30, 2);
function after_entry_created($entry_id, $form_id){
  // Referat 1
  if($form_id == 2){                           //2 is the ID of my registration-form
    global $wpdb, $frmdb;
    $reward_id = $_POST['item_meta'][130];     //130 is the ID of your drop-down data from entries field in your reservations-form
    $seat_count_field = 127;                   //127 is the ID of the available seats field in the class-form
    $available = FrmEntryMeta::get_entry_meta_by_field($reward_id, $seat_count_field, true);
    $wpdb->update($frmdb->entry_metas, array('meta_value' => ((int)$available-1)), array('item_id' => $reward_id, 'field_id' => $seat_count_field));
  }

Idea:
Decrease an available count in another form
This example assumes you have two forms set up: one for the event and one for reservations. The event form will include a field for the number of available openings. The reservation form will include a data from entries field to list all the events. This function will drop the number of available openings for an event when it is selected.

I’m sorry but I’m not programmer enough to find the problem myself.
Many thanks for your great help.