Just hit the question mark ("?") button on the top right of the diplomatic scroll before making an agreement with the faction that your diplomat is talking to. It almost always works unless you're asking for something the game cannot give (such as money beyond the faction's means in exchange for a ceasefire, trade rights, towns and cities, map information, etc.)
It's nothing happening when I click the ? in the diplomacy menu, that's the problem
Step by step:
1) Prepare the deal u want, then
2) Click on the "?", then
3) click on "Send proposal" or whatever is the button called.