Some leaders are willing to negotiate again faster than others (mainly the more peaceful personalities).
"lets stop this pointless war" means offering peace at even terms (no cities, techs or gold change hands).
If an AI feels like they are winning, or have more power than you then they prolly won't accept this.
Cities under attack (or with a stack going there) matter for such negotiations, we can see this when they want city X for peace.
(or for those who watched AI Survivor..when you facepalmed about cities given back
War isn't the only possible reason why an AI does not talk, if you gave in on a "stop trading with our worst enemy" demand you didn't just cancel deals with them - they are now also bored of you for x turns and refuse to talk.