Okay, so Japan denounced me, and sent lots of troops to my border. No problem. Then he started sending missionaries to my cities, and I demanded that he stop. He promised not to but did not stop. So I went to denounce him, and found I could not. Option not there. Hmmm, maybe because he already denounced me? No idea really. But there is an option for causus belli, to declare war formally... Okay, I wait one turn to finish the encampment. Next turn, the causus belli has disappeared, all I can do is declare a surprise war. But now I can denounce him. I can't imagine the rule that would legitimately explain this. I suppose his denouncing me expired that turn -- but that rule would be having a ridiculous result. You should always be able to denounce, even if it is, for the moment, superfluous.