on the checkout failure page, add the following code to dump the session:
<pre><?php var_dump($_SESSION); ?></pre>
the session will contain the response from the gateway with details on why it is failing.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.