Adnan's regressed significantly since his breakthrough season with us. Don't know why he wanted a loan to Dortmund when they had all those stars in the first 11 and some decent options on the bench already; his time at Dortmund was a waste. The point of a loan at his age back then was to get game time. Case in point, Andreas Pereira with Granada this past season. That's as good a loan as we've seen from our players in quite a while. He played a few different positions, played for a team that suited him, and most importantly, he played a lot of games.
And then Adnan goes to Sunderland and we all know how woeful they were. He couldn't get in that team by the last few weeks! And that's with a manager who used to be able to get the most out of him before. Maybe the fame got to his head, and it's easy to see why. Imagine this, you're 18, barely anybody recognises you. And then you play a lot of games for one of the biggest clubs in the world, you produce for that team almost week in, week out. People want to take photos with you, they say you're the best young player in town since Ronaldo, they start comparing you to other legends who have already make their mark in world football.
The icing on the cake is you get a massive bump in your wages, and suddenly your life looks a LOT easier. You could buy things you could only dream of before, you could spend and spend and not worry much, if at all. And after all that, what's the point of even trying to improve? You could coast through life. This is where mentally, I think most people (not just footballers, mind you) cannot overcome. The feeling of having made it; you're on top of the world, might as well enjoy it, right? Why put in more effort?
I don't know how true/real his attitude problems were, but when LVG, Tuchel, Jose and freakin' Moyes question your attitude, commitment and work ethic, and the common denominator in all cases is Adnan, you'd have to wonder if the lad ever wanted to be more than the boy who could do anything (for 7 months).
Going back to your question as this is getting rather long by a tad bit, I think the biggest thing about transfers that some people never consider is leverage. I'll give you 2 examples. First one, us with Pogba last year. He just had a phenomenal season with them, especially considering his age. He was one of their key players, their secret weapon. Juve didn't want to sell him, they almost made it all the way in UCL, they didn't need to sell him. They were trying to build a formidable team, and they wanted him to be a key part of that. They had everything in their power to suck us dry, and they did (from their point of view at least. I still think we got him for a bargain if he reaches his full potential, but thats a story for another day).
The second example is Ronaldo when he left us. We had some leverage. Our team back then was really good, Ronaldo was our key player, and he had extended his contract with us the year prior. But that was offset by his desire to join Real. It was quite known that it was always his dream to play for Real. And Madrid of course knew that, thus them using that to their advantage in the negotiations.
Back to Adnan. He's regressed dramatically since his breakthrough season, he's reportedly not put in the effort desired by 4 managers, he couldn't help his last team avoid relegation, he only has a year left with us. We have almost no leverage to ask for a lot of money! Right now, he's a risky signing, at best for most clubs. And risky signings don't go for silly money.
Wow that sure was a lot. Sorry! This is what happens when you're bored at work, and your supervisors have already left for Eid holiday.
Hopefully some of the things I said made sense.