Getting AI settlements to rebel can really be a pain. Preventing rebellion is one of the few things the AI is good at. They will lower taxes, train garrisons, and even sometimes move their capital in order to prevent it. In my experience, the most important thing you should do when trying to encourage rebellion is to destroy their MIC's (and keep them from being repaired), in order to prevent them from training a larger garrison. Timing is also important. Make sure they have as few troops as possible in the city, and check to make certain they don't have many troops nearby that could reinforce the garrison either. If you can do that, from my experience you can usually get a city to rebel after a few turns of spying and sabotage. I'm willing to bet that Ankyra has a pretty big garrison by now. If so, I'd recommend you pull all your spies and assassins back for a few years, and wait for most of the garrison to leave before renewing your espionage assault.