Knocking won't work. He will ignore you completely until you have a diamond. For now, you have to acquire a certain tool that will help you obtain one.

In Gobliiins, you always have to finish everything you have to do on a given screen. When you do, a "GO" button appears and you move to the next screen. You haven't done quite enough yet. Hint: notice there's something waving in the wind. Hint 2: all three goblins have a task to perform in this screen.

The codes are a save system, effectively. You get one when you click the "GO" button and you should write it down. The last three letters of the code record the state of your health bar, which is replenished only once during the game, so it's a good idea to figure out how a scene is played, inevtiably losing some health in the process, and then go through it again with clean score.