First, check the eCart Checkout button server behavior on the cart.php page.
In the Server Behavior panel, double click the eCart Checkout button server behavior and make sure it is pointing to the checkout page.
if it is still not working, there may be a php error occurring that is causing the redirect to fail, add the following code to the checkout page at line1 to turn error reporting on:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>