Help with Azir late late game

Hey guys, I just started ranked this season I for some reason I have decided to main Azir. I have played about 20 games with him so far and I love him. However I have noticed that when I start getting to late game, I spike in power a lot in the early part of the late game, but if the game goes on to the late late game, like when everyone has a full build or is close to it I have a harder time doing the damage I did earlier. Why do you think this is? I know azir is supposed to be awesome late game so I know I'm doing something wrong. It varies but I usually go nashors> rylas>boots>ludens or zhonyas>abysal scepter or void staff.