I signed Origi on loan for Newcastle and barely played him. He asked to cancel loan and now his long term plan is does not want to play for Newcastle. When I signed him I told him he would be squad rotation but I treat him like a back up. My assumption was he thought I broke my promise on playing time and that's why he won't sign for the club. If he signed for me again I might tell him he will be in the first team then leave him on the bench.
If you told him he was a key player but you only play him fairly regular this might be the problem. If he was willing to sign for you once then maybe he will again but probably not if he isn't getting enough game time. Try playing him more and maybe praise him when you get chance. Otherwise you'll just have to sell him and buy someone.