to be fair, I think that "it's all on me" is more a statement that Fleck is the head coach, so everything is ultimately his responsibility - even though there are coordinators and assistant coaches who handle most of the day-to-day coaching, call the plays, and work the X's and O's. I don't think Fleck is saying that he's directly responsible for players being out of position, or missing tackles.
And, it's hard to tell who's at fault without being at practice. For all we know, the coaches are telling the players over and over again that, "against this formation, your read is X, and your responsibility is Y," and the players just aren't doing what they're supposed to be doing. it could be mental mistakes. it could be physical mistakes - it could be lack of physical ability.
And it's hard to know - without being in the locker room - what adjustments are being made. All we can do is watch the game, and it looks like the same players making the same mistakes. So, either no adjustments are being made, or they are making adjustments, and the players are simply incapable of executing what needs to happen.
So, Fleck is in a bind. he can't say "we told Cornerback X what to do 27 times, and he still did the wrong thing." Even if it's true, you can't do that as a Coach. And he can't say "my coaches can't make adjustments," even if it's true.
sometimes, the best thing to say is, "the other team was better today," and just leave it at that.