The other factor when it comes to demands is your relative military strengths. In general, your advisors have to say "compared to them, we have a strong military!" to even have a chance of getting a city or resource (or 5 gold, for that matter) from an AI. And, as eveyone else has stated, if it's their only source, or a city with a resource, there's no chance they'll cave. It may get them mad, but they won't declare on you. The best way to get them to declare is if you get them furious (if they aren't already) and THEN ask them to "leave or declare!" If you check out some of the succession games, they will attempt to trap military units (scouts and workers won't do it), get them furious, and then ask them to leave, in the hopes of getting a war declaration, and the resulting war happiness.