Managing projects, training people, and several other improvement and control activities involve a cycle. At one point, you observe something like "this person came unprepared to the meeting." You may or may not attempt to "move the needle" when you make this observation. Later, you have another observation like "we didn't get the contract" or "the meeting wasn't able to settle the items that were scheduled". You see the connection between these two. The junior may not.
If you go to the person and only tell them your first observation, especially before the "bad thing" has happened, and suggest changing their ways, they may or may not act on it. It is far more powerful to go to them with the second observation, and help them to connect it back to the first.
That means when something has gone wrong, you need them to see why and decide for themselves what behaviour to change. Sure, to you it's obvious when you see the "junior" behaviour happening, you know what it will lead to, but they do not and your word alone won't make that connection. You've tried a word or two in private conversation "don't do X again" but without success.
Assuming this really is a role you're supposed to take on, and that neither the junior nor their manager will be upset that you do it, you might try introducing them to the idea of 5 whys.
Sidenote on 5 whys:
It was pioneered at Toyota and used in assorted "quality" programs around the world. Working out why a project was late (or failed), a client was lost, or an accident happened is a very difficult thing, and 5 whys is a profoundly useful technique for solving these things.
It is not about condescending or treating them like a child or forcing them to admit their mistakes. It is about working together to discover the reasons something happened. You may go into this believing you know why (and you may be right) but going through the process together is important.
It might go like this:
Hey, I wanted to take a few minutes to talk to you about the slip on the Smith project. That was a pretty big deal.
Have you heard of a technique called 5 whys? You basically ask why something happened, and then whatever the reason, ask why on that, and so on. Let's try it. Why did we have to take that big slip?
[junior says "because the X wasn't ready" or "because we found too many whatevers" or "because the client wouldn't agree at the acceptance meeting". If they shrug and don't know, or say "Because Steve is a moron" then you lead them to the very simplest literal first reason why the bad thing happened. Because whatever wasn't ready.]
So why was that?
[we weren't given enough time, the spec was incomplete, Steve is a moron, whatever. Again lead them a little if you need to. For example if they say "nobody did x in advance" you might need to say "I believe it was you who was supposed to x. So perhaps the issue is that nobody told you that you needed to. Why didn't you do it? Did you know it needed to be done? Did you wonder who was doing it?" The idea is questions questions questions, "why" to everything instead of stopping at "nobody told me to do that" or "they were just angry" and pushing through to root causes. Ideally the person learns that what they do matters.]
And why wasn't the P finished before the R was started? Because you chose to get started on the R without waiting for approval.
And why wasn't the client satisfied? Because you didn't bring the right things to the meeting to show them.
At this moment they see the connection between the behaviour (not preparing properly for the meeting, or not taking the meeting seriously) and the result (schedule slip, client lost, whatever.) Sit in that for a few seconds. And then, this is incredibly important do not stop there at the "oh crap I screwed up a big project the slip is all my fault" sensation finally reaching your junior. It's important they feel this, but you must not stop here.
And why were you left to make a decision that could ruin the whole project? [Or, and why were you not aware that preparation for that meeting was so important?, or and why were you not aware of what Steve needed from you and when? etc]
Because I didn't give you the advice (or because nobody gave you the advice) I am here to give you today. These meetings matter. You need to be ready for them. If we blow the meeting, the whole project can end up slipping. I guess you've sort of learned that one the hard way. Let's not have to learn it again, ok?
You look sympathetic. You look "this is not all on you" while not letting go of "some of this is on you". You draw a clear and obvious path from one choice of theirs (bad priorities, missing a communication, not preparing for something) to a bad outcome that has clearly and unequivocally happened. You reassure them that part of the responsibility is on the company or department for not teaching them this connection before, but emphasize that they are now aware of and responsible for this connection. Now they know. It can't happen again. And then you smile and conclude the meeting.