Reasons it might not flip...
1. You don't have enough culture there (you need more culture on that tile than any other civ). Doesn't appear to be the problem in this case.
2. Revolt check keeps failing. There's only a 10% chance of checking for a revolt on any given turn, so you could just get unlucky over and over.
3. Too strong of a garrison / too weak of a revolt. Beyond a certain size, garrisons will simply suppress revolts. E.g., sticking 9 archers in Dortmund in the screenshot above would keep it from flipping until the Medieval Era; using 13 archers could keep it secure until the Renaissance, and so forth.
4. You could have accidentally selected the "no city flipping" option in game options. Check your game settings page to confirm that's not the problem.
5. If they "conquered" it from you (which includes cities you gift to them; I'm not sure if it includes cities that culturally flipped to them), it won't flip unless you selected the "city flipping after conquest" option.
6. If you ran an Incite Revolt espionage op there, that prevents it from getting the "warning" revolt (before flipping, cities always get one "warning" revolt which does not flip). If it never gets what the game considers a "warning" revolt, it can never flip.