Chat models only work within a limited context window. In long conversations, early details get dropped or deprioritised. New chats also start blank unless a memory feature carries things over, and that feature saves only selected facts.
To reduce forgetting, keep one chat per task and restate key facts when a thread gets long. Keep a short reusable brief with your role, audience, format and constraints, and paste it at the start. Use the custom instructions or project features for things that never change, and check the memory settings to see what is saved. For long documents, ask for a summary of the decisions so far and begin a new chat with that summary. Check the tool's current help pages for how memory works.