custom built script works great on my domain, but not others.

Hello, I have a custom Magento 1.7 extension (installed and working on several websites) which I tried installing on a client's domain. It was expensive to have developed, so I had my previous developer set up ionCube for licensing. 3 months later, I finally made my first sale... but the script doesn't function correctly when I installed it on my client's domain... and I can't get a hold of the original developer to help with it. (He's the one that originally installed it.) So I tried installing it directly from source code, but no dice. I'm not a PHP guru, so it could be something simple. I do know it works perfectly on the other domains it was installed on, so I can show you the working installations. I have ALL the source code AND a working install. I need you to troubleshoot the broken installation. Login credentials will be provided for you, so you can troubleshoot both working and broken installations. Once it's working correctly, I would like a suggestion on which licensing software to use (ionCube/Zend/etc.) -- whatever works the best! Let me know if you have any questions. Thanks!