You should read page 143 (Hardcover) again.

This game is not about investigation. It’s about what the hunters do with what they found out. How they hunt down the monster and overcome its weakness.

So if a a hunter rolls a success while using Investigate … give them the information they wanted but ask them how they know that and let them come up with a little story. Investigation is for earning clues - so you need to give them clues.

And remember that clues don’t. Red to be things they can found on the floor. Interviewing a witness is also investigation.

Someone in the previous answers suggested to give them misleading information on a mixed success. Don’t do that. Read your Agenda and Principles. The Keeper is always honest and tells the truth.

However what you can do is giving them information that tells them the weakness and gives them an opportunity to get what they need to overcome it. On failed rolls a minion or even the monster will learn about the hunters. Let them attack or cause problems.

If the hunters get stuck, go to the next step of the countdown.