If you are getting the "Transaction not completed" message on your web page, then add something like:
<?php var_dump($_SESSION); ?>
it will have lots of information, but included will be the paypal response and reason why it failed. Once you have that text we will be able to help you diagnose the problem further.
If you have eCart you can access the response from paypal in your bindings panel and drag and drop the error description individually onto the page.