I don’t understand why I can’t respond to responses from my feed?

You can't reply directly from your news feed, you have to go to the actual thread. You can get there by clicking/tapping the tile of the thread, it should be in blue.

It's kind of difficult but you can respond to the original response to you, just not to responses to that, so go back to the original response and respond to IT and the response will just line up as "1 more reply". Hard to tell you, but sometime I just make a new note above with typing in the name of the person.