Assistant only citing one file from VectorStore?

Using file_search with GPT-3.5 Turbo, I first upload my files (i.e. 3 PDFs), then create a VectorStore. I then pass the VectorStore’s ID to a call to create an Assistant with file_search.

Over around a dozen trials, they all seem to only be citing (pulling data?) from one of the PDFs. Any reason why this would be?

Would I have to prompt the Assistant somehow to ensure that it pulls from all 3?