That's great to hear! I think it might be some settings you can enable on the tenant rather than completely changing the tenant to the US. We are using it on our tenant in the EU (Nordic), and it works fine. I remember that our admin configured and enabled some settings in the tenant before we start to work with copilot studio.
This link may have instructions for the solution. Please check it out if you want to try making the EU tenant work with Copilot: https://learn.microsoft.com/en-us/copilot/microsoft-365/microsoft-365-copilot-enable-users
I've experienced similar issues with Copilot struggling to understand the context and content of my documents, particularly with complex structures.
One approach that improved its performance for me was creating better prompts. Additionally, I found that using the "Topic" function to divide complex documents into different sections or topics helped significantly. This approach seems to aid the model in chunking the documents and understanding the content more effectively.
You can also create specific triggers and prompts for these topics, which can help the model locate the relevant content more accurately.
This might be a current limitation of Copilot compared to models like GPT-4 or Gemini, which tend to handle complex documents more effectively.