Data not retrieved from database

I have this piece of code:

<?php
global $wpdb;

$table_name = $wpdb->prefix . 'wpex_programma';
// This retrieves the data from the database
$retrieve_data = $wpdb->get_results( "SELECT Anaam FROM {$table_name}" );
?>
<form action="#" enctype="multipart/form-data" method="post">
    <?php wp_nonce_field( 'set_programma_action', 'set_programma' ); ?>
    <table>
        <?php foreach ( $retrieve_data as $retrieved_data ) { ?>
            <tr>
                <th>Programma:</th>
                <td style="vertical-align: middle;"><?php echo esc_html( $retrieved_data->Anaam ); ?></td>
                <th>
                    <button name="programma" type="submit" value="<?php echo esc_attr( $retrieved_data->Anaam ); ?>">Abonneer</button>
                </th>
            </tr>
        <?php } ?>
    </table>
</form>

<?php
    // Verify nonce and safe the data if the user is logged in.
    // Nonce docs: https://developer.wordpress.org/themes/theme-security/using-nonces/
    if (isset( $_POST['programma'] ) && isset( $_POST['set_programma'] ) && wp_verify_nonce( $_POST['set_programma'], 'set_programma_action' )) {

      $data = filter_input( INPUT_POST, 'programma', FILTER_SANITIZE_STRING );
      $current_user_id = get_current_user_id();

        if ( $current_user_id && ! empty( $data ) ) {
            update_user_meta( $current_user_id, 'programma', $data );
        }
    }
?>

The page does not return the data form the database. Nothing is shown except my navbar and footer. How can I solve this problem?

The code for retrieving the data and displaying it is:

global $wpdb;

$table_name = $wpdb->prefix . 'wpex_programma';
// Dit haalt de data op uit de database
$retrieve_data = $wpdb->get_results( "SELECT Anaam FROM {$table_name}" );
?>
<form action="#" enctype="multipart/form-data" method="post">
    <?php wp_nonce_field( 'set_programma_action', 'set_programma' ); ?>
    <table>
        <?php foreach ( $retrieve_data as $retrieved_data ) { ?>
            <tr>
                <th>Programma:</th>
                <td style="vertical-align: middle;"><?php echo esc_html( $retrieved_data->Anaam ); ?></td>
                <th>
                    <button name="programma" type="submit" value="<?php echo esc_attr( $retrieved_data->Anaam ); ?>">Abonneer</button>
                </th>
            </tr>
        <?php } ?>
    </table>
</form>

I will also show my database structure:

My wpex_usermeta, the choices have to be saved here:

My wpex_programma. All the programs are saved here:

EDIT

echo var_dump($wpdb->prefix);

yields

string(5) "wpex_"

I also did echo var_dump($tablename); and it showed NULL