Problem: Textbox doesn't fire TextChanged event after some text is entered?

I have a form (.aspx) that has multiple controls like dropdownlist, textbox (textbox1), ...

The original dropdownlist outpost is true and when I change the selected index of it, depending on which index I selected, the text is written in textbox 1, and then since textbox1 autorepeat is true, 2 other textboxes are filled based on the text written in textbox 1.

But surprisingly, when I inject some text directive into textbox 1, the text changed event doesn't fire.

why?

I am using Asp.net 3.5, C #.