The impression I got, and I could well be wrong on this, is that De Bruyne is happy to come but doesn't want to request a move and forsake a loyalty payment (or possibly tarnish his reputation with their fans should it fall through).
Wolfsburg, for their part, reluctantly accept that they can't really turn down a bid of that magnitude (despite the backing of VW), but don't want to be seen to be forcing him out of the door. It would anger their fans, and force them to pay the aforementioned "loyalty" bonus.
Thus, it's a stalemate at the moment hence our reluctance to make a formal bid at present. One that I'm pretty sure will be resolved sooner rather than later though, as it's not insurmountable.