Have you tried expanding the instructions INSIDE the function? Like the description field? I use that extensively to describe how to use the functions and what to do with the parameters. I also use Description for certainly parameter fields where it makes sense.
Furthermore if anything I can recommend to look very critically at your Assistant prompt. It is probably not detailed and clear enough. And I’m saying that based on my own experience doing the same that you are doing. I have a several assistants running that have 10 functions and a prompt that is more than 2 pages long. And I still keep updating the prompt every week.
If you had to explain a 10 year old to do what you want the assistant to do - would he or she be able to do it? Would I understand your prompt if I read it without knowing anything else? I have come to believe we’re at best mediocre at giving clear instructions 