Turkish Airlines Is Not Changing My Flight Date (FlexFly)

I have a flight between May 20, 2025, and May 30, 2025, and my ticket is Flex, meaning it is changeable. However, I need to change the dates, but I can't. Neither online nor by phone—every time I call Turkish Airlines, they say the same thing: there is a system error, please try again in a few hours, but when I try again a few hours later, I get the same response. I even tried reaching out through Turkish Airlines mobile app and Twitter support, but no one offered a real solution. I have a visa appointment next week, and I need to adjust my application accordingly. How is this supposed to work? I don't understand.