Copilot Agent always respond with fallback topic

Question

I've created an agent on Copilot Studio, but suddenly, it's only responding through Fallback topic. I've uploaded several PDF files, on the knowledge base, in order to train the agent, but somehow, it stopped to consult them. I tried to create a new agent from scratch, uploaded the PDF files again, but it didn't worked. Tracking the question, it always goes to fallback topic, and if I disable it, the agent says that there's no topic related to the question. Can someone help me please?

Thanks!