They also won't take your Open Borders if their approach towards you is negative (WAR/HOSTILE/DECEPTIVE/GUARDED), or they think you're setting a trap for them. This happens because "Demand" is still using the same trade window as "Trade". I'm hoping this can be fixed one day, but as I understand it it'd require some complicated and difficult UI work. In the meantime, I removed the mechanic where a failed demand activates the cooldown timer where you can't ask again for X turns. The AI might agree to give items that you demand for if the deal value is IMPOSSIBLE, including World Congress votes.