Why I can't intervene a war against a rival?

I tried to intervene a war against 2 of my rivals 3 times already, each try after I got a new diplomat, after that I don't get any pop-up or message about it, my diplomat just goes somewhere and that's it, am I missing something? Country on the other side is kinda losing so I expect them to let me help them, in case they need to accept or decline intervention