Turkish Airlines Flight Change Issue

I purchased round-trip tickets for two from Izmir to Baku via the Turkish Airlines app, opting for the top package that allows refunds and changes. Later, I was informed that the date of one of my flights had changed. Since it didn't suit us, I wanted to postpone it by a week. We had planned a short trip of two to three days, preferring early and short waits; my daughter is young, and I don't want to wait long at airports. They couldn't change the flight to the desired time (I didn't understand the reason) and the call center also told me I couldn't select this flight. I had to choose another flight. However, later in the app, I noticed I could select the desired flight, but they were asking for a $300 fare difference. If I can purchase that ticket and I was inconvenienced due to your canceled flight, and if I can make the desired change as per what I was told, why is the 08:55 flight not suitable for me but the 12:00 flight is? If I can buy the 08:55 flight later by paying the fare difference (meaning my tariff allows it), why wasn't the change made initially? My husband works there, I work here, and it's very difficult to get leave. I bought the best package (the top package for economy class) so that my daughter could see her father a few hours earlier and to be able to cancel or change in case of any situation. This seems like a method to extract extra money from people. I also wrote it as feedback but got no result. They said, 'We already made a free change,' but this was not the change I wanted. Not offering a selectable flight and demanding $300 under the name of fare difference within the package is really unbelievable. I hope we can find a solution soon; my flight is on March 21st.