Increase the number of variations loaded in the WooCommerce front end for dynamic filtering of available variations in the drop down lists.

increase-variation-threshold.php

<?php //only copy this line if needed

/**
 * Function filters the threshold for the amount of variables to load in the front end.
 *
 * @see    https://woocommerce.wordpress.com/2015/07/13/improving-the-variations-interface-in-2-4/  
 *
 * @return int 100 The new threshold.
 */
add_filter( 'woocommerce_ajax_variation_threshold', function() { return 100; } );