Copilot Access not working for only 1 user

It looks like a permission issue.

Your user can open the agent, so they have access to it. The problem is that they almost certainly don't have access to the content it reads. Copilot agents are security-trimmed: they only answer from files the signed-in user can open. If the user can't read the training documents, the agent behaves exactly as you describe (where it simply says it found nothing.)

I'd check the following:

1. Ask the end-user to Test the document access directly. If the user can access the document then the issue coulbe be in the Copilot permission
2. Check how the agent was shared. It's likely here where the issue happens. Check if the permission was shared to the specific user or was it a group permission? either way, go to step 3.
3. Re-share the agent to the user.