After hours of searching i found the solution in my problem. The problem was in ajax_variation_threshold. I had to increase the limit of the requests. So i had to add in my functions.php this code:
function woo_custom_ajax_variation_threshold( $qty, $product ) {
return 50;
}
add_filter( 'woocommerce_ajax_variation_threshold', 'woo_custom_ajax_variation_threshold', 10, 2 );
this seems to work properly.