My GPT keeps randomly picking a document in its knowledge section to summarise, rather than the one I attach

I Have my own Custom GPT But I’m encountering an issue. That is very frustrating. I have provided my GPT with some background knowledge, including some documents in its knowledge section. I have asked it to summarise a document that I attached to the chat and ask it for insight etc . However, it randomly picks another document in its knowledge section to review. This is, despite me referencing the name of the attachment in the prompt. I have discussed the issue with the GPT builder and tried lots of variations of the prompt so I don’t think it is user error however, I am very open to be incorrect and educated! The only way I can get it to work is to go to the main chat GPT and ask it there. This it can do, but is not optimal as it does not possess the background training. I have given my custom one.

Does anyone know what I’m doing wrong or is this a wider problem with custom GPT?

I would really appreciate any help can give me as this is very frustrating.

Thank you.