Found this:
http://www.reddit.com/r/civ/comments/1fxrbe/how_to_demand_from_those_pesky_ais_and_succeed/
I can't confirm the correctness of this, but the poster seems pretty certain of their statements
Here's how to get AIs to accept your demands:
Conditions where AI will never accept no matter the circumstances:
Too soon (20 turns + a random number between 1-10) turns since last demand
You're unforgivable (tons of red negative modifiers) to the AI
AI is Hostile toward you
AI military stronger than human's
AI is far/distant away and the human doesn't have troops on their home front
There are two numbers. Value willing to give up, and odds of giving in.
Initial odds based on AI boldness. Formula is (10 - Boldness) * 10. So 5 boldness = 50% initial chance.
Afraid (+50 odds, +200 value)
Military Strength:
Strong (+10 odds, +120 value)
Powerful (+20 odds, +200 value)
Immense (+35 odds, +200 value)
In total, the most you can demand is 400 value, although realistically your best chance is 200 value (immense military).
Consult the chart linked above to determine the odds of a demand going through. Game rolls a random number. If that number is smaller than the odds of giving in, then AI has possibility of accepting.
AI value chart:
Gold: maximum of either the gold you requested, or (gold requested * 10 / AI gpt). Essentially, if I request 237 gold, and the AI has 3 GPT, the actual value of the demand is 237 * 10 / 3 = 790 requested value. If I do the same thing and the AI has 20 gpt, then I've only requested 118 value.
GPT: (gpt requested * duration * 80 / 100). If I request 10 gpt, then on Standard that becomes 10 * 30 * 80 / 100 = 240 value.
Resources: +200 per luxury, +(40 * number requested) per strategic (5 iron = 200 value)
Open Borders = 50 value
If the value demanded > value willing to give up, then the AI requests. If not, then they accept.
Maybe this will clear things up. These are the only conditions the AI will accept demands in. Nothing else gets factored in.