Tips on how to recreate GPT using assistant API

Hey! I am trying to create a sales assistant, and actually did manage to get really good results in the GPT builder. The problem is that whenever I try to achieve the same results by using assistants API, the assistant seems so much more stupid. It has the same files and settings, however it recommends random products and talks BS.

Should I fine tune my model? Why is to much different? Please provide any tips or links to documentation, I am really lost here ;/