A combination of injuries which blighted his first season and a new manager coming in that didn't fancy him in his second season that saw him play so little football at the club. I have wondered would Mancini have persevered with him had he remained in charge, by all accounts he was a huge fan of Jacks.
No doubt his move to City affected his development one way or another but this is not exclusively a City issue, it can happen to any player that moves to a club with greater competition for places, especially when said player is injury prone.