I have several instances of source (Native) text. Once I've translated one of them all other were propagated automatically. Then I've changed translation, but it wasn't propagated to other instances - what should I do?

Such behavior is needed for the cases if you've translated all instances of some source text
in one way (e.g. they were autotranslated as duplicates) and then you need to translate only several of them in other way - in this case translations won't be overwritten where they shouldn't.

However, if you need to overwrite translation of all instances - open Productivity Expert (Experts window appears along with Translation window when you click on some row), there's dropdown button "Propagate all" - open it's dropdown list and check "Overwrite translations", then click "Propagate all".