Not detecting that media is already in library

Hi there, I have been trying in vain to register only new images in the database, but the plugin doesn’t seem to be able to identify when an image is already registered in the library. It will say it’s not registered when it is, and i have been able to register the same image 4 times. On another (very different) test site the plugin works perfectly, but not here. I have tried the following:

• Switching themes
• Disabling all plugins
• Removing any htaccess rules that might cause issues
• Enabled error logging in wp-config- no errors from this plugin to be found
• Used the network tab in chrome- it only returns saying the product registrations were successful.

Have you any ideas on what could be causing this? I’ve run out of ideas so any assistance would be greatly appreciated ??