Why sometimes, even if I have more then 50% culture in a tile, I do not gain control of it ? I conquered Spanish Sagunto as France in 1800s (it was Spain's city since spawn) but didn't have ability to conquer rest of their cities so made peace. I used great artist to culture bomb city, and now some of tiles next to it have 55-70% of my culture, but are still Spanish.
I remember similar thing happening after conquering Mediolanum as Germany or some north Ethiopian cities as Egypt.