Even in the side they way they've played he should have been able to stand out in his own performances (and taking his long injury lay off into account) but he simply hasn't as you say shone which he should have been able to do regardless of the players around him.
It seemed before when he was younger it was down to the way he applied himself re his weight/fitness etc and just question whether he really has that true desire to push himself to the level required on a constant basis?
As I say he has the talent but there's something lacking in him.

His weight and application only seems to be a problem at FC. There must be more to it than something lacking in him.

Players sometimes flourish in different environments.

His weight & fitness hasn't been an issue for a long while but it took a while for him to address it which has now. However at SL level something is missing as he has the talent but just can not seem to take the opportunity when he gets the chance and stamp himself on a game and show he can play at this level. He's not played SL level at any club other than Hull FC so if its not down to application then maybe he just isn't a SL standard player? He seems to have the talent but can not put that into action so far in his career at SL level.

His weight and fitness problems were immediately resolved during his loan spell wit Rovers.

They weren't immediately resolved at Rovers. He'd already openly admitted he hadn't done enough over the years in terms of his weight & fitness and that he liked his junk food way before he'd gone on loan. So it's nothing to do with either Hull FC or Hull KR, it was due to his lack of application of which he admitted that much.

Get him signed up so you can have him assisting while you scrat around for SL surviva yet again next season.

From the bits and pieces I've read, Abdull's issue is that he needs to be playing. When he isn't, his focus drifts and and he gets down. As he's down the pecking order at Hull, and unlikely to get much game time when your more senior halfbacks are fit he wants to move to play more. I don't he's trying to screw Hull over - he just wants to play.

So he just wants to play, and if he doesn’t he’ll sulk.
The thing is, he’s had ample opportunity to make himself undroppable as we’ve lost our first three half backs but he hasn’t taken his chance.

It's not necessarily sulking - he just wants to go elsewhere, probably recognizing that at this stage in his career and without more chances to develop, he's unlikely to displace your incumbent halfbacks.