In hindsight, I think he should have won for 11/12 but the award is given out before the end of the season so the Agueerrrroooooo moment wasn't considered.
In 2012/13, when he was fit, he was at his physical best beating players with his pace (something he hasn't really done for about 4 years now) but he kept trying to blast it iirc, and missed an unusual amount of chances for him.
In 13/14, I think David Silva or Yaya should have won although it's equally difficult to look past Suarez given the season he had. Dzeko's brilliance also took the limelight off Serge.
In 14/15, another good chance to win it but then again Hazard was class too. After that - until maybe this season, he's been good but not really in the reckoning for the award.
I think if any City player has been robbed, it's probably Vincent Kompany in 10/11. He was part of the best defence in the league, easily the best defender in the league (probably the league's best ever defender) plus no attackers were setting the league on fire as the top scorers only got 20 goals and the league was won with only 80 points. He also captained us to the FA Cup and Champions League qualification for the first time.