in the WA_eCart/scripts/copyShipping.js file, there are two arrays created. One for the billing form element names, the other for the shipping form element names:
var billingFields = new Array('firstname', 'lastname', 'email', 'phone', 'fax', 'street1', 'street2', 'city', 'state_province', 'postcode', 'country');
var shippingFields = new Array('shipping_firstname', 'shipping_lastname', 'shipping_email', 'shipping_phone', 'shipping_fax', 'shipping_street1', 'shipping_street2', 'shipping_city', 'shipping_state_province', 'shipping_postcode', 'shipping_country');
the first thing to check is to make sure the two state_province form eleemtns are correctly listed here.
If you still having problems, please send a link where I can see the issue